jail: Add prison_isvalid() and prison_isalive()

prison_isvalid() checks if a prison record can be used at all, i.e.
pr_ref > 0. This filters out prisons that aren't fully created, and
those that are either in the process of being dismantled, or will be
at the next opportunity. While the check for pr_ref > 0 is simple
enough to make without a convenience function, this prepares the way
for other measures of prison validity.

prison_isalive() checks not only validity as far as the useablity of
the prison structure, but also whether the prison is visible to user
space. It replaces a test for pr_uref > 0, which is currently only
used within kern_jail.c, and not often there.

Both of these functions also assert that either the prison mutex or
allprison_lock is held, since it's generally the case that unlocked
prisons aren't guaranteed to remain useable for any length of time.
This isn't entirely true, for example a thread can assume its own
prison is good, but most exceptions will exist inside of kern_jail.c.