autoconf support for libsystemd-daemon stuff
systemd PID-tracking and fast-restart hacks

This uses the systemd APIs combined with a few odd hacks
to accomplish two goals:
  1) Accurately deliver the MAINPID to systemd without
     using pidfile-contents stuff (which would be racy
     and/or break our flock() mechanics).
  2) Support gdnsd's fast-overlapped-restart via ExecReload,
     (which means real reloads have to be done externally
     since we've repurposed ExecReload, but that works ok
     with just "/usr/sbin/gdnsd reload").

The hacky bits are these:
  1) Because $NOTIFY_SOCKET isn't handed to "control" processes
     (e.g. ExecReload), we set it manually to the standard value
     "@/org/freedesktop/systemd1/notify" before calling sd_notifyf().
  2) Because a descendant of our "control" process from ExecReload
     is going to become the new MAINPID (with notification!), it
     has to promote itself from "control" process to a main process
     before dropping root privs.
     This is accomplished by writing our PID directly to
     "/sys/fs/cgroup/systemd/system.slice/" + sd_pid_get_unit(getpid())
     + "/cgroup.procs".

Surprisingly, this works, at least with systemd-208 on Fedora20.

This is the unit file that worked with this code (note especially
the Service section's first few lines, those are important!):
---------------------------------------------------
[Unit]
Description=gdnsd
After=local-fs.target
After=syslog.target
After=network.target

[Service]
Type=forking
NotifyAccess=all
GuessMainPID=false
ExecStart=/usr/local/sbin/gdnsd start
ExecStop=/usr/local/sbin/gdnsd stop
ExecReload=/usr/local/sbin/gdnsd restart

[Install]
WantedBy=multi-user.target
---------------------------------------------------
Merge branch 'dmn-rework' into systemd

Conflicts:
	gdnsd/libgdnsd/libdmn/dmn.c
Merge branch 'dmn-rework' into systemd

Conflicts:
	gdnsd/libgdnsd/libdmn/dmn.c
Merge branch 'dmn-rework' into systemd
Merge branch '2.x' into systemd
Merge branch '2.x' into systemd

Conflicts:
	configure.ac
move cgroup hack up to end of fork, before extmon execl()
Merge branch '2.x' into systemd
Various systemd improvements

1) Only try to be systemd friendly if it's booted *and*
   we're not flagged foreground, *and* stdin is not a tty
2) Only set MAINPID early if we're really going to kill
   a previous instance during a restart via ExecReload.
Merge branch '2.x' into systemd
Merge branch '2.x' into systemd
Merge branch '2.x' into systemd
Split systemd basic support from reload hacks

(And clean up various related autoconf bits, etc)
Use sd_journal_printv for all log output under systemd

(also, removes some of the previous stderr hacks that were
just for extmon, as they make things ridiculously complicated)
systemd watchdog timer stuff, abstracted via libdmn

(also, fix early dmn_log ordering during init1)
fixup for ifdef name confusion in 1f245437
Merge branch '2.x' into systemd
fix --with-systemd-hax autoconf stuff
Fix stilly error output issues with 0330f2b5

(was hidden by earlier autoconf screwup in local testing)