For safety, ensure that any consumer of the set_regs() and
ptrace_set_pc() use the correct return to userspace using iret.

The signal return, PT_CONTINUE (which in fact uses signal return path)
set the pcb flag already.  The setcontext(2) enforces iret return when
%rip is incorrect.  Due to this, the change is redundand, but is made
to ensure that no path which modifies context, forgets to set
PCB_FULL_IRET.

Inspired by:	CVE-2014-4699
Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week