add support for building libtls

Use './configure --enable-libtls' to build the library and install the
associated manpages. Note that the API and ABI of this library may
change still, though feedback is welcome.

ok deraadt@ jsing@ tedu@
update changelog for libtls
simplify packaging for libtls.

Derive the file list from the Makefile.am template itself (DRY), ensure
manpage is packaged with the final tarball.