jail: separate "statically valid allow flags" from "prison0 allow flags"

[kevans]

Summary

The current setup means that we can't add an allow flag in sys/jail.h
that's disabled by default without removing it from the pr_flags
assignment in kern_jail.c. That's technically fine, but I think it's
better to make it more prevalent at the PR_ALLOW_* definition site so
that it's top-of-mind when adding a new flag.

This is a preparatory change for adding an allow flag that prison0 will
also have disabled by default, but with an allow.* knob and sysctl to
enable it.