bsd.files.mk: Fix FILESPACKAGE

Because TAGS may already have a package tag, assigning TAGS to
FILESTAGS causes the value of FILESPACKAGE to be ignored.

Only assign tags other than package, and take the default package from
${PACKAGE}, which matches the behaviour of bsd.confs.mk and bsd.man.mk.

This fixes several files that were wrongly installed in utilities,
including /usr/share/examples/sendmail/mailer.conf and some files
in /usr/share/nls.

The fix trips a bug in share/syscons where Makefile.inc was setting
FILESPACKAGE to the wrong package, but the problem was hidden because
the subdirectories set PACKAGE explicitly. Fix this by setting the
correct FILESPACKAGE in Makefile.inc and removing the PACKAGEs.

In stable/14, this creates a new FreeBSD-examples package, so add an
entry to Makefile.packages.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D51784

(cherry picked from commit 1e629ebb15db9ee0cb11a3654d7bcd3046a0df54)