Fix GH-15061

Reported at https://hackerone.com/reports/3288162

This underflow does not occur in Ruby because:

* This function is `static` and not accessible other than from ruby
  internal.

* Ruby uses mode 0 when calling this function directly.

* For `%f` in vsnprintf.c using mode 3, this parameter comes from the
  precision, but negative precision is meaningless and ignored.

The spinlock here performs poorly when there are multiple Ractors. The
improvement on single threaded performance doesn't seem that
significant, so I think we should just use malloc.

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)