Style changes:
- Move two IOCTL related defines to the top of the C-file
- Add more comments describing the recently added IOCTL small size and
small align macros
Assert the state of the process lock and sigact mutex in
kern_sigprocmask() and reschedule_signals().

Discussed with:	rea
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week