dhclient(8): Enable numbered user class ID option

By adding it to the option priorities table.

PR: 184117
Submitted by: Lowell Gilbert <freebsd-bugs-local at be-well.ilk.org>
Reported by: Tomek CEDRO <cederom at tlen.pl>
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D7911