Update/fix gtk-doc support
- No obvious mistakes in the code
- updates libtasn1's gtk-doc support files from gtk-doc-tools 1.28,
- fixes some issue in the current setup (indent fix, diverging prototypes)
- adds some missing documentation.
env DOC_MODULE=libtasn1 gtkdoc-check
will still not succeed since this would require that everything was documented. I think doing this properly would necessitate to change the long list of #defines constants to a couple of enum the current state would become to wordy.