The SSR1 register can have bits set that don't match to bits in the MSR

The SSR1 register can have bits set that don't match to bits in the MSR
register.  We would set the sc_ps member of struct sigcontext to SSR1
when entering a signal handler, and compare it to PSL_USER in sigreturn(2)
to make sure that the user code didn't set any bits it shouldn't set.
If non-MSR bits are set that comparison would fail and sigreturn(2) would
fail.  Fix this by initializing sc_ps to PSL_USER instead of taking its
value from SSR1.  On OpenBSD we always run processes with the same MSR
value.

ok deraadt@