vmon: introduce vmon utility

vmon exposes the monitoring overlays of vwm through a standalone
commandline utility by creating dedicated window for presenting the
overlay.

At this time a single preexisting PID may be specified, forming the root of
a recursively monitored heirarchy.  Alternatively, a command may specified
which vmon will then fork and execute on your behalf, automatically
monitoring the child and its descendants for you, in a style similar to
strace.

Examples:

 Monitor a linux kernel build in fullscreen mode, note the --:
 `vmon --fullscreen -- make -C /usr/src/linux -j8`

 Monitor the entire system:
 `vmon --fullscreen --pid 1`

 For convenience, omitting --pid and a command to run assumes PID 1:
 `vmon --fullscreen` is analogous to `vmon --fullscreen --pid 1`

 Monitor a linux kernel build fullscreen at 50Hz:
 `vmon --fullscreen --hertz 50 -- make -C /usr/src/linux -j8`

 Do the same thing but don't exit when the make completes:
 `vmon --linger --fullscreen --hertz 50 -- make -C /usr/src/linux -j8`

 Help is provided via `vmon --help`, where you'll find all flags described
 with their short and long forms.

Some important TODO items include:

 - Support for specifying multiple top-level processes

 - Support for mixing --pid and running a command
   (useful for watching specific system processes while you're running
    something specific)

 - Support for scrolling within the window.  The overlays in general need
   to evolve a bit to better support the vmon use case.  In vwm there
   wasn't any intention of accomodating the entire space if it exceeded
   what was naturally available in the existing window's dimensions.
   That makes sense for vwm, but vmon not so much.

   You can achieve the same thing more or less by resizing the window to be
   larger than the screen (easy to do in vwm using a combination of
   Mod1-Right-Click to resize, then Mod1-Left-Click to drag the window,
   repeatedly.  Then just Mod1-Left-Click to grab the window and "scroll"
   it by moving the desired part on-screen, repeatedly.  Cumbersome, but
   works fine in a pinch.  Not all window managers can do this though...

   Of course it would be less costly to only render what's visible, like a
   scrollable window would achieve.  This is probably the top priority
   TODO.

 - Support for monitoring of memory use, files, per-process IO activity.
   libvmon supports substantially more than is being visualized currently.

 - Support for changing the font.

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