pkgbase: Add some tags to files installed in distribution target

Add the MK_MAIL dependant file to the runtime package as well as the
MK_KERBEROS ones the empty locate database, the FreeBSD copyright file
and the GENERIC.hints.
Tag the unbound link from /etc to /var to belong in the unbound package.

Reviewed by: bapt
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D20607