When the registerred unblock function is called, it should retry
the cancelled blocking function if possible after checkints.

For example, `SIGCHLD` can cancel this method, but it should not
raise any exception if there is no trap handlers.

The following is repro-code:

```ruby
require 'socket'
PN = 10_000

1000000.times{
  p _1
  PN.times{
    fork{
      sleep rand(0.3)
    }
  }
  i = 0
  while i<PN
    cpid = Process.wait -1, Process::WNOHANG
    if cpid
      # p [i, cpid]
      i += 1
    end

    begin
      TCPServer.new(nil, 0).close
    rescue
      p $!
      exit!
    end
  end
}
```

	[Bug #20592] Fix segfault when sending NULL to freeaddrinfo

	On alpine freeaddrinfo does not accept NULL pointer

	Fix dangling `else`

Make sure the correct error is raised for EAI_SYSTEM resolver fail

	In case of EAI_SYSTEM, getaddrinfo is supposed to set more detail in
	errno; however, because we call getaddrinfo on a thread now, and errno
	is threadlocal, that information is being lost. Instead, we just raise
	whatever errno happens to be on the calling thread (which can be
	something very confusing, like `ECHILD`).

	Fix it by explicitly propagating errno back to the calling thread
	through the getaddrinfo_arg structure.

	[Bug #20198]
	---
	 ext/socket/raddrinfo.c | 25 +++++++++++++++++++------
	 1 file changed, 19 insertions(+), 6 deletions(-)

Remove setaffinity of pthread for getaddrinfo

	It looks like `sched_getcpu(3)` returns a strange number on some
	(virtual?) environments.

	I decided to remove the setaffinity mechanism because the performance
	does not appear to degrade on a quick benchmark even if removed.

	[Bug #20172]
	---
	 ext/socket/extconf.rb  |  2 --
	 ext/socket/raddrinfo.c | 48 ++++--------------------------------------------
	 2 files changed, 4 insertions(+), 46 deletions(-)

	Free pthread_attr after setting up the thread

	[bug #20149]
	---
	 ext/socket/raddrinfo.c | 12 +++++++++++-
	 1 file changed, 11 insertions(+), 1 deletion(-)

The document must be placed immediately before the class definition.
No other statements can be placed in between.

Again, rsock_raise_socket_error is called only when getaddrinfo and getaddrname fail

According to https://bugs.openjdk.org/browse/JDK-8268605, pthread_create
may fail spuriously. This change implements a simple retry as a modest
measure, which is also used by JDK.

Do not use `pthread_attr_setaffinity_np` if `sched_getcpu()` exceeds
`CPU_SETSIZE`. (Using `CPU_ALLOC()` would be more appropriate.)