Now, calling `rb_fork` directly breaks the PID cache and the timer
thread, so must use `rb_fork_ruby` or similar instead.

RB_OBJ_WRITE already exists in rgengc.h, so we shouldn't redefine it in
gc.h.

* process.c: Add Process._fork

This API is supposed for application monitoring libraries to hook fork
event.

[Feature #17795]

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

These contents are purely implementation details, not worth appearing in
CAPI documents. [ci skip]

Allow the 'Dir.home' method to reliably locate the user's home directory when
all three of the following are true at the same time:

    1. Ruby is running on a Unix-like OS
    2. The $HOME environment variable is not set
    3. The process is not a descendant of login(1) (or a work-alike)

The prior behavior was that the lookup could only work for login-descended
processes.

This is accomplished by looking up the user's record in the password database
by uid (getpwuid_r(3)) as a fallback to the lookup by name (getpwname_r(3))
which is still attempted first (based on the name, if any, returned by
getlogin_r(3)).

If getlogin_r(3), getpwnam_r(3), and/or getpwuid_r(3) is not available at
compile time, will fallback on using their respective non-*_r() variants:
getlogin(3), getpwnam(3), and/or getpwuid(3).

The rationale for attempting to do the lookup by name prior to doing it by uid
is to accommodate the possibility of multiple login names (each with its own
record in the password database, so each with a potentially different home
directory) being mapped to the same uid (as is explicitly allowed for by
POSIX; see getlogin(3posix)).

Preserves the existing behavior for login-descended processes, and adds the
new capability of having Dir.home being able to find the user's home directory
for non-login-descended processes.

Fixes [Bug #16787]

Related discussion:
    https://bugs.ruby-lang.org/issues/16787
    https://github.com/ruby/ruby/pull/3034

To fix build failures.

Devs do not love "3".  The only exception is RUBY3_KEYWORDS in parse.y,
which seems unrelated to our interests.

This shall fix compile errors.