libnvmf: Add nvmf_nqn_valid_strict() function

This moves the checks previously under #ifdef STRICT in
nvmf_nqn_valid() into a separate helper for userland. This
requires that the NQN starts with "nqn.YYYY-MM." followed by at
least one additional character.

Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D48767