Rework PRIVATELIB

Now when a lib is marked as PRIVATELIB it is renamed into libprivate$foo instead
of being installed in /usr/lib/private and playing with rpath.

Also allow to install headers for PRIVATELIBS in that case the headers will be
installed in /usr/include/private/$foo

Keep the headers under a private namespace to prevent third party build system
to easily find them to ensure they are only used on purpose.

This allows for non base applications to statically link against a library in
base which is linked to a privatelib

Treating PRIVATELIBS as regular libraries allows to push them into our current
compatX packages if needed.

While here finish promotion of libevent as PRIVATELIB
Install header for bsdstat and libucl

Differential Revision: https://reviews.freebsd.org/D2365
Reviewed by: brooks, des
Discussed with: imp