MFC r272069:
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.
MFC r272070:
Expand the libthr(3) manpage to document knobs accepted by libthr.so
and explain some internal working of the library, neccessary to
understand the knobs effects.

MFC r272153 (by pluknet):
Fix description of mutex acquisition.