Some minor code refactoring in v4l2 plugin

Small code changes to decrease indentation a little.
Fix a possibly left open file descriptor in v4l2 plugin
Use macro for prefixing log messages in v4l2 plugin
Add more logging to v4l2 plugin.

This adds some mostly user focused logging statements.
Add fallback sizes and framerates to v4l2 plugin.

Until a proper fix is found to support devices that use stepwise
or continuous values for framesize and framerate this adds fixed
values that can be selected.
For devices that support discrete values those are still queried
and used.
Use default color space in v4l2 plugin
Merge pull request #180 from fryshorts/v4l2-input

Assorted fixes for the v4l2 plugin