[LITMUS^RT] Doxygen support for liblitmus

[Björn Brandenburg]

Hi everyone,

Roy has kindly documented the public API using Doxygen. Thanks, Roy!

I've merged the patch into the staging branch.

	https://github.com/LITMUS-RT/liblitmus/commit/33efb47a220a3edfcb80ddf7aa20d65c26ebf916

To test this, run 'make doc' in the latest version of the staging branch, which will create a docs/ directory with HTML documentation. 

Three points to consider:

1) If you  have pending changes to liblitmus, please be aware that this will likely result in nasty merge conflicts because the Doxygen touches the code base in many places. If this is a problem, please raise the issue now.

2) I'd appreciate if future patches come with Doxygen-enabled comments.

3) There are some @todos in the comments. Patches addressing these would be very welcome. This is nice and easy low-hanging fruit, if anyone wants to get into LITMUS^RT development.

Thanks,
Björn