_exit(2), execve(2): cancel per-process interval timers safely

During _exit(2) and sometimes during execve(2) we need to cancel any
active per-process interval timers.  We don't currently do this in an
MP-safe way.  Both syscalls ignore the locking assumptions documented
in proc.h.

The easiest way to make them MP-safe is to use setitimer(), just like
the getitimer(2) and setitimer(2) syscalls do.  To make things a bit
cleaner I have added a helper function, cancelitimer(), so the callers
don't need to fuss with an itimerval struct.

While we're here we can remove the splclock/splx dance from execve(2).
It is no longer necessary.

ok deraadt@

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