Don't document test cases

Issue #121 changed filenames for test cases. The file exclusion pattern
has to be updated accordingly.
Add create_signal_number_set to pselect_api

The pselect function requires a signal number set argument. pselect_api
has to provide a way to create a signal number set.
Fix broken links in documentation
Merge pull request #139 from magicant/chore/doxygen

Miscellaneous fixes about doxygen