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

[ivy]

Summary

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.

MFC after: 1 day

[emaste]

etc/mtree/BSD.usr.dist

17–26

gmock and gtest ought to be moved to a tests package

[ivy]

etc/mtree/BSD.usr.dist

17–26

yes, i already put this on my TODO list :-)

i think they should be in a separate gtest package, which matches how we handle atf and kyua.