man: use configured path for mount and umount binaries in manpages

Export the MOUNT_PATH and UMOUNT_PATH variables as XML entities and use them in
the systemctl.1 manpage instead of hardcoding the path in /usr/bin.

Tested:
- Ran ./configure ac_cv_path_MOUNT_PATH=/bin/mount (same for umount) and
  rebuilt the manpages, confirmed that the correct path was in man/systemctl.1
- Rebuilt man/systemd.directives.xml and the man pages derived from it,
  confirmed that the correct paths were there as well.

Makefile.am

@@ -6505,6 +6505,8 @@ substitutions = \
        '|DEBUGTTY=$(DEBUGTTY)|' \
        '|KILL=$(KILL)|' \
        '|KMOD=$(KMOD)|' \
+       '|MOUNT_PATH=$(MOUNT_PATH)|' \
+       '|UMOUNT_PATH=$(UMOUNT_PATH)|' \
        '|MKDIR_P=$(MKDIR_P)|' \
        '|QUOTAON=$(QUOTAON)|' \
        '|QUOTACHECK=$(QUOTACHECK)|' \

man/systemctl.xml

@@ -413,8 +413,8 @@
           processes. Not all unit types manage processes of these
           types however. For example, for mount units, control processes
           are defined (which are the invocations of
-          <filename>/usr/bin/mount</filename> and
-          <filename>/usr/bin/umount</filename>), but no main process
+          <filename>&MOUNT_PATH;</filename> and
+          <filename>&UMOUNT_PATH;</filename>), but no main process
           is defined. If omitted, defaults to
           <option>all</option>.</para>
         </listitem>