etc/mtree: Add package tags for /usr/include

Set the default package to clibs-dev, since that's where the vast
majority of include files comes from. Add explicit package tags
for the directories which are installed in their own package.

Approved by: re (cperciva)
MFC after: 1 day
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53577

(cherry picked from commit 436618a427b4baaf42d8221ef07d14e3ba787d3a)
(cherry picked from commit 736771f631557213269692f3a5611a1f0bdab2bd)