zsh completion: more correctly omitting closing -p quote

If I have

$ sysdig -p '%fd.si

and I press TAB, I do not want

$ sysdig -p '%fd.sip'

Note the trailing '

This patch fixes this. Previously I switched to _describe from _alternative to
get this behavior, but that doesn't actually work (why did I think it did?).
We're now good except some pieces of the code use _describe and others use
_alternative. Would be good to be consistent, but it really doesn't matter
zsh completion: added new options in 0.1.85
bash completion: added new options in 0.1.85
Merge pull request #204 from dkogan/master

Tab completion updates