Fix leak reported by Valgrind when setting environment variables

[peterzhu2118]

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)