Pull up following revision(s) (requested by apb in ticket #26):
	sys/kern/subr_prf.c: revision 1.156
If mutex_tryenter() fails, don't call mutex_exit().
Pull up following revision(s) (requested by apb in ticket #27):
	build.sh: revision 1.295
Only the -m command line option, not MACHINE from the environment,
is supposed override MACHINE_ARCH from the environment with the
default MACHINE_ARCH for the requested machine.
Add and use an opt_m variable to enforce this.
Tickets 26, 27