pkgbase: share rules for packages without pkgdeps

Have liby and libcompat share *-dev and *-lib32_dev rules, and share
*-man rule for libcompat and libelftc. Also correct name substitution
and description for man rules.

Reviewed by: manu
Fixes: 5391bcf0f7e4 ("pkgbase: Do not record dependency on...")
Fixes: 65fa2fd23b9d ("pkgbase: Do record dependency on non-...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37964