Update to doxygen 1.8
Check for socklen_t (suggested by g.esp and Stefanos Harhalakis)
Hide undocumented in auto-generated documentation.
Use @retval for correct man page formatting.

When using @return instead of @retval, -1 formats as the beginning of
emphasis in the man pages.  The need for this was deduced from the
fix-manpages patch to debian libnet 1.1.6+dfsg-1.
Fix gcc warnings.

This is just to make -Werror succeed as the samples are not that
important.