Switch to libcap-ng, avoid linking libdbus against libcap[-ng]

Switch to libcap-ng, avoid linking libdbus against libcap[-ng]

(Commit message written by Colin Walters <walters@verbum.org>)

A current Fedora goal is to convert projects to libcap-ng which
more easily allows dropping Linux capabilities.  For software
which also links to libdbus, it's problematic to link against
libcap as well.

Though really, libdbus should have never linked against libcap
in the first place, which is another thing this patch changes
by moving the libcap-using bits out of dbus/ and into bus/.

https://bugzilla.redhat.com/show_bug.cgi?id=518541