refcount(9): Add refcount_release_if_last() and refcount_load()

The former is intended for use in vmspace_exit(). The latter is to
encourage use of explicit loads rather than relying on the volatile
qualifier. This works better with kernel sanitizers, which can
intercept atomic(9) calls, and makes tricky lockless code easier to read
by not forcing the reader to remember which variables are declared
volatile.

Reviewed by: kib, mjg, mmel
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27056