Move wavemonrc to XDG_CONFIG_HOME

[grrtrr]

Support for this was started in #105, but is not uniform yet.

The following needs to be resolved:

• support for the XDG base directory specification,
• gracefully handle existing ~/.wavemonrc (e.g. by unlinking or moving it),
• update documentation,
• make naming consistent.

[grrtrr]

Implementation Plan

1. When writing the configuration file

Writing is easier, since it can use the new location right away.

1. Determine BASE:
   • if XDG_CONFIG_HOME is set, use this value as BASE,
   • else, set BASE = ~/.config (as per specification);
2. If the BASE directory does not exist, create it.
3. If the BASE/wavemon directory does not exist, create it.
4. Use BASE/wavemon/wavemonrc for the location of the wavemon runtime-configuration file.

2. When reading the configuration file

1. If ~/.wavemonrc exists, read it, but unlink(2) it afterwards.
2. Else use BASE/wavemon/wavemonrc as above.

3. Notes

• The name/path was chosen for compatibility with the existing wavemonrc(5) man-page.
• Man-page(s) and documentation need to be updated accordingly.