Switch the defaults to not split the RLIMIT_STACK-sized initial thread
stack into the stacks of the created threads.  Add knob
LIBPTHREAD_SPLITSTACK_MAIN to restore the older behaviour.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272069 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f