nagstamon.py: Add some more fallback path checking

Maybe I'm reinventing pkg_config here, but I believe the increase
user friendliness outweighs that.

Signed-off-by: Anton Lofgren <alofgren@op5.com>
ninja: merge Ninja2 & Ninja3 servers

The differences are small enough that it's hardly hurtful to keep
them in a single class. This should make things a bit easier for
users who are not entirely sure what version they're running.

Signed-off-by: Anton Lofgren <alofgren@op5.com>