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>
Revert "build-sys: gettexts 0.18 -> 0.18.2 due to MKDIR_P"

This reverts commit 0576dbd3ea114dc35b797ad7216f1ed5814e7c6b.

There is two possible ways:

 1/ autotools complains that gettext 0.18 uses deprecated macro
 2/ users complain that 0.18.2 is too new and they cannot rebuild

We care about users and project contributors, so let's live for the
next util-linux release with the old stupid gettext 0.18.

Signed-off-by: Karel Zak <kzak@redhat.com>
fstrim: daily -> weekly in systemd timer

 * add link to fstrimg man page
 * less aggressive frequency

Signed-off-by: Karel Zak <kzak@redhat.com>
fstrim: add hint to man page

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