Add `Xsunaba` option parsing

[morgant]

Currently, Xsunaba references environment variables for overriding configuration values. This was just quicker & easier to implement as Xsunaba is intended to accept a command and its parameters to be be executed. I'd like to add more configurability to what options are passed to Xephyr(1) (especially the -resizable option; see issue #3), more automated audio support (see issue #4), etc.

So, I'd like to add option parsing for the current functionality with respect to the environment variables, retaining support for the environment variables. Specifically, Xsunaba should parse options before -- as its own and anything after as the command to run (and its parameters).

[morgant]

I have started implementing this using getopts. It doesn't support X-style long options (e.g. -geometry 1024x768), so I'm not trying to parse out some options and let others pass through to Xephyr and instead just implementing specific options, currently:

• -h: built-in help
• -v: verbose output
• -r: Xephyr -resizeable
• -G: Xephyr -no-host-grab
• -d <display>: Use X host display
• -s <width>x<height>: Xephyr -screen <width>x<height>
• -u <username>: sandbox username

Currently, I'm just overriding the existing XSUNABA_* environment variables that I was already supporting, so they still "just work".