merge revision(s) r44712,r44715,r44716,r44722,r44725,r44726,r44753: [Backport #9454] [Backport #9828]

	* thread_pthread.c: get current main thread stack size, which may
	  be expanded than allocated size at initialization, by rlimit().
	  [ruby-core:60113] [Bug #9454]

	* thread_pthread.c: rlimit is only available on Linux.
	  At least r44712 breaks FreeBSD.
	  [ruby-core:60113] [Bug #9454]

	* thread_pthread.c (ruby_init_stack, ruby_stack_overflowed_p):
	  place get_stack above others to get stack boundary information.
	  [ruby-core:60113] [Bug #9454]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46468 b2dd03c8-39d4-4d8f-98ff-823fe69b080e