Add memnotify plugin for signaling memory pressure level changes

If /dev/memnotify is available the plugin will set up two memory use
triggering levels. These are used to determine normal/warning/critical
memory pressure situations. The current state can be queried via D-Bus
method call "get_memory_level" and changes are broadcast as D-Bus signal
"sig_memory_level_ind".

Introspect data for new signal and method call is:
  <node name="/com/nokia/mce/signal">
    <interface name="com.nokia.mce.signal">
      <signal name="sig_memory_level_ind">
        <arg name="memory_level" type="s"/>
      </signal>
    </interface>
  </node>

  <node name="/com/nokia/mce/request">
    <interface name="com.nokia.mce.request">
      <method name="get_memory_level">
        <arg direction="out" name="memory_level" type="s"/>
      </method>
    </interface>
  </node>

While it is up to other components to react to changes in memory
pressure, it is expected that on warning level non-essential caches
should be released and on critical level non-essential processes
should be terminated.

As the actual level limits must device specific, the built in config
does not specify warning/critical levels.

The levels can be specified by installing configuration files to
"/etc/mce/<priority><name>.conf". Where priority is two digit
number from 00 to 99 (60 is used for other device specific configs)
and name should be something unique enough (say, package name).

The levels can be adjusted during mce runtime via mcetool options
      --set-memuse-warning-used=<page_count>
      --set-memuse-warning-active=<page_count>
      --set-memuse-critical-used=<page_count>
      --set-memuse-critical-active=<page_count>

The mcetool --status option shows the current memory use level.

[mce] Add memnotify plugin for signaling memory pressure level changes. Fixes JB#22490
Fix compilation warning from fake_owner_gone() function

Since fake_owner_gone() is no longer a callback, the parameter no
longer needs to be of abstract gpointer type.
Unify mcetool --status output format

All values except brightness fading values have trailing colon ':'
in mcetool --status output.

Add one for brightness fading values too.
Tune memnotify logging levels

Too verbose devel time logging was accidentally left in place.

Remove some debug logging altogether and demote others to sane verbosity
levels so that on default verbosity level only real failures are logged.
Merge pull request #212 from spiiroin/add_memnotify_plugin

Add memnotify plugin