Manual merge changes from the release-2.x.y branch into default.

- Easy change in certificate.h (conflict with new doc style).
- Easy change in ssl-gnutls.c (keep my huge debug statement and comment
from 2.x.y)
- I didn't copy any changes from de.po. I always assume translation changes
should not be propagated between the release-2.x.y branch and default.
Translators are responsible for updating both of their translation files
as desired.
- I completely dropped datallah's change to certificate.c from
https://hg.pidgin.im/pidgin/main/rev/2948449ffd12
The purple_certificate_display_x509() function no longer exists in default.
It was changed in https://hg.pidgin.im/pidgin/main/rev/d65261e7426e
and then removed in https://hg.pidgin.im/pidgin/main/rev/cf4c54e487bc
I might check whether we should make datallah's change in the new code in
default.