build-sys: consolidate systemd support

 * systemd (since v209) uses only one library (when compiled
   without --enable-compat-libs)

 * all systemd build-sys stuff is merged into HAVE_SYSTEMD
  (automake) and HAVE_LIBSYSTEMD (C macro) now

 * all is controlled by --with-systemd, default is to automatically
   check for systemd libs

 * no more --enable-socket-activation and --enable-journald

Signed-off-by: Karel Zak <kzak@redhat.com>