Implement LIBADD

LIBADD will automatically set DPADD and LDADD when needed including their
dependencies, LIBADD automatically handles private and internal libs so that
the end user Makefile does not have to care about it.

This allows to reduce overlinking on the base system leaving the framework get
the dependencies properly.

It also allows to built components binaries statically.

To use it:

Replace:
DPADD=	${LIBARCHIVE} ${LIBSSL}
LDADD=	-larchive -lssl

by:
LIBADD=	archive ssl

Differential Revision:	https://reviews.freebsd.org/D1209
Reviewed by:	brooks imp
Correctly specify -L for internal and private libs