provide bash tab completion for gnutls-cli, gnutls-serv, and certtool

the gnutls binary utilities are useful, but i often need to read the man pages to get them to work right.

they'd be even more useful if they provided bash tab completion.

https://github.com/scop/bash-completion has a bunch of examples, but gnutls has so much internal structure and knowledge about its arguments and what are availalble that it would make sense to ship it from gnutls directly.

sorry i don't have time to work on it now, just thought i'd record the feature request here!