libpam: Don't put man8 in the dev package

These are user-facing manpages, but were installed in the runtime-dev
package since the PAM modules use bsd.lib.mk. Use MANNODEV instead of
MAN to put them in the base runtime package instead.

Approved by: re (cperciva)
Fixes: 031e711647c3 ("packages: Install development manpages in the -dev package")
MFC after: 3 days
Reviewed by: bapt
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53601

(cherry picked from commit b1972bd9a03331f97823b93c3d1be74e7373b07a)