RFC: API change for sema_timedwait

[John Polstra]

Before 5.x becomes -stable, I'd like to change the API of
sema_timedwait(9).  This function is used in only 3 places in the
kernel, all in "dev/ips/ips_commands.c".

Currently, sema_timedwait returns 0 if the operation fails due to a
timeout.  On success, it returns a non-zero value.  This is precisely
the opposite of the standard convention in the kernel, where 0 means
success and a non-zero value (taken from <sys/errno.h>) means failure.
The convention exists because most functions can succeed in only one
way but can fail in several different ways.

The reason I care about this is because I'd like to add new functions
sema_wait_sig() and sema_timedwait_sig() which can be interrupted
by a signal.  Then sema_timedwait_sig could fail in two different
ways: as a result of a timeout or as a result of a signal.  If these
functions returned proper errno values on failure, it would be easy to
distinguish between the two failure cases.

This change would also make the return values of sema_timedwait,
sema_wait_sig, and sema_timedwait_sig consistent with the analogous
condition variable operations cv_timedwait, cv_wait_sig, and
cv_timedwait_sig and with tsleep and msleep.

Does this change sound OK to you folks?

John