The absence of either the integer or fractional part should be
allowed.

We need to keep a pointer to the originally allocated environ because
adding more environment variables can cause it to be changed to something
else.

For example:

    100.times do |i|
      ENV["FOO#{i}"] = "1"
    end

Causes Valgrind to report:

    312 bytes in 1 blocks are definitely lost in loss record 9 of 13
      at 0x484D444: calloc (vg_replace_malloc.c:1340)
      by 0x1884F8: calloc1 (gc.c:1844)
      by 0x1884F8: objspace_xcalloc (gc.c:12202)
      by 0x1884F8: ruby_xcalloc_body (gc.c:12209)
      by 0x4204DD: ruby_init_setproctitle (setproctitle.c:119)
      by 0x27DDF4: ruby_process_options (ruby.c:3101)
      by 0x160BD1: ruby_options (eval.c:117)
      by 0x15B96E: rb_main (main.c:40)
      by 0x15B96E: main (main.c:59)

The call to setenv replaces the string in the environ array, which causes
the original string to be reported as a memory leak.

This commit allocates another copy of environ called alloc_environ that
contains the strings allocated by ruby_init_setproctitle. Then in
ruby_free_proctitle it frees all the strings in alloc_environ.

For example:

    ENV.each { |k, v| ENV[k] = v.dup }

Valgrind reports:

    3,321 bytes in 39 blocks are definitely lost in loss record 3 of 3
      at 0x484884F: malloc (vg_replace_malloc.c:393)
      by 0x25CB5B: objspace_xmalloc0 (gc.c:11972)
      by 0x40344E: ruby_strdup (util.c:540)
      by 0x59C854: ruby_init_setproctitle (setproctitle.c:143)
      by 0x38CC44: ruby_process_options (ruby.c:3101)
      by 0x234DB1: ruby_options (eval.c:117)
      by 0x15B92E: rb_main (main.c:40)
      by 0x15B92E: main (main.c:59)

It is undefined behaviour to free environ as it is managed by the system.
This caused RUBY_FREE_AT_EXIT to double free on systems like Linux. This
commit changes it to only free orig_environ, which is enough to make
both Valgrind and macOS leaks tools to not detect memory leaks.

The environ is malloc'd, so it gets reported as a memory leak. This
commit adds ruby_free_proctitle which frees it during shutdown when
RUBY_FREE_AT_EXIT is set.

    STACK OF 1 INSTANCE OF 'ROOT LEAK: <calloc in ruby_init_setproctitle>':
    5   dyld                                  0x18b7090e0 start + 2360
    4   ruby                                  0x10000e3a8 main + 100  main.c:58
    3   ruby                                  0x1000b4dfc ruby_options + 180  eval.c:121
    2   ruby                                  0x1001c5f70 ruby_process_options + 200  ruby.c:3014
    1   ruby                                  0x10035c9fc ruby_init_setproctitle + 76  setproctitle.c:105
    0   libsystem_malloc.dylib                0x18b8c7b78 _malloc_zone_calloc_instrumented_or_legacy + 100

when the RUBY_FREE_ON_SHUTDOWN environment variable is set, manually free memory at shutdown.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Peter Zhu <peter@peterzhu.ca>