Set default stack size for Linux apps to 8MB. This matches Linux'

Set default stack size for Linux apps to 8MB.  This matches Linux'
defaults, makes core files smaller, and fixes applications which use
pthread_join(3) in a wrong way, namely Steam.

This is based on a patch submitted by Jason Yang, which I've reworked
to set the limit instead of only changing the value reported (which
is enough to fix the bug for Linux pthreads, but could be confusing).

PR:		248225
Submitted by:	Jason_YH_Yang at wistron.com (earlier version)
Analyzed by:	Alex S <iwtcex@gmail.com>
Reviewed by:	emaste
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D26778