Since it's a singleton method, it should be referenced as `Socket.tcp_fast_fallback=`,
not `Socket#tcp_fast_fallback=`.

This change includes the following updates:
- Added an environment variable `RUBY_TCP_NO_FAST_FALLBACK` to control enabling/disabling fast_fallback
- Updated documentation and man pages
- Revised the implementation of Socket.tcp_fast_fallback= and Socket.tcp_fast_fallback, which previously performed dynamic name resolution of constants and variables. As a result, the following performance improvements were achieved:

(Case of 1000 executions of `TCPSocket.new` to the local host)

Rehearsal -----------------------------------------
before   0.031462   0.147946   0.179408 (  0.249279)
after    0.031164   0.146839   0.178003 (  0.346935)
-------------------------------- total: 0.178003sec

            user     system      total        real
before   0.027584   0.138712   0.166296 (  0.233356)
after    0.025953   0.127608   0.153561 (  0.237971)

with `Socket.tcp_fast_fallback=`
The functions that `Socket.tcp` had are now also available in `TCPSocket.new`.

* Do not save the last_error if there are no sockets waiting to be connected

In this implementation, the results of both name resolution and connection attempts are awaited using select(2).
When it returned, the implementation attempted to check for connections even if there were no sockets currently attempting to connect, treating the absence of connected sockets as a connection failure.
With this fix, it will no longer check for connections when there are no sockets waiting to be connected.

Additionally, the following minor fixes have been made:

* Handle failure of getsockopt(2) and removed unnecessary continue in the loop

* Tweak: Use common API to check in_progress_fds

* Safely call TCPServer.new in test

* Set empty writefds when there is no socket waiting to be connected

* Enable fast_fallback option

to suppress failing output in CI.

* Introduction of Happy Eyeballs Version 2 (RFC8305) in TCPSocket.new

This is an implementation of Happy Eyeballs version 2 (RFC 8305) in `TCPSocket.new`.
See https://github.com/ruby/ruby/pull/11653

1. Background
Prior to this implementation, I implemented Happy Eyeballs Version 2 (HEv2) for `Socket.tcp` in https://github.com/ruby/ruby/pull/9374.
HEv2 is an algorithm defined in [RFC 8305](https://datatracker.ietf.org/doc/html/rfc8305), aimed at improving network connectivity.
For more details on the specific cases that HEv2 helps, please refer to https://bugs.ruby-lang.org/issues/20108.

2. Proposal & Outcome
This proposal implements the same HEv2 algorithm in `TCPSocket.new`.
Since `TCPSocket.new` is used more widely than `Socket.tcp`, this change is expected to broaden the impact of HEv2's benefits.
Like `Socket.tcp`, I have also added `fast_fallback` keyword argument to `TCPSocket.new`.
This option is set to true by default, enabling the HEv2 functionality.
However, users can explicitly set it to false to disable HEv2 and use the previous behavior of `TCPSocket.new`.

It should be noted that HEv2 is enabled only in environments where pthreads are available.
This specification follows the approach taken in https://bugs.ruby-lang.org/issues/19965 , where name resolution can be interrupted.
(In environments where pthreads are not available, the `fast_fallback` option is ignored.)

3. Performance
Below is the benchmark of 100 requests to `www.ruby-lang.org` with the fast_fallback option set to true and false, respectively.
While there is a slight performance degradation when HEv2 is enabled, the degradation is smaller compared to that seen in `Socket.tcp`.

```
~/s/build ❯❯❯ ../install/bin/ruby ../ruby/test.rb
Rehearsal --------------------------------------------------------
fast_fallback: true    0.017588   0.097045   0.114633 (  1.460664)
fast_fallback: false   0.014033   0.078984   0.093017 (  1.413951)
----------------------------------------------- total: 0.207650sec

                           user     system      total        real
fast_fallback: true    0.020891   0.124054   0.144945 (  1.473816)
fast_fallback: false   0.018392   0.110852   0.129244 (  1.466014)
```

* Update debug prints

Co-authored-by: Nobuyoshi Nakada <nobu.nakada@gmail.com>

* Remove debug prints

* misc

* Disable HEv2 in Win

* Raise resolution error with hostname resolution

* Fix to handle errors

* Remove warnings

* Errors that do not need to be handled

* misc

* Improve doc

* Fix bug on cancellation

* Avoid EAI_ADDRFAMILY for resolving IPv6

* Follow upstream

* misc

* Refactor connection_attempt_fds management

- Introduced allocate_connection_attempt_fds and reallocate_connection_attempt_fds for improved memory allocation of connection_attempt_fds
- Added remove_connection_attempt_fd to resize connection_attempt_fds dynamically.
- Simplified the in_progress_fds function to only check the size of connection_attempt_fds.

* Rename do_pthread_create to raddrinfo_pthread_create to avoid conflicting

---------

Co-authored-by: Nobuyoshi Nakada <nobu.nakada@gmail.com>

[Misc #18891]

:resolv_timeout of TCPSocket.new is not implemented for now.

Add connect_timeout to TCPSocket.new in the same way as Socket.tcp.

Closes [Feature #17187]