German translation update

--HG--
branch : release-2.x.y
Allow incoming IQ stanzas with an empty 'from' if they're in response to
an outgoing stanza to our bare or full JID. Patch from Thijs Alkemade
from https://developer.pidgin.im/ticket/15879

Fixes #15879

--HG--
branch : release-2.x.y
Change strings from "help on a specific command" to "help with a specific command"

I think the former is ok, but I think the latter is better.
Add string hint for translators.
Standardize the formatting we use to document hints for translators.
Add note for translators.
Add note for translators.

Also, why do we have this?? This is totally not functionality that anyone
needs.
Add note to translators.
More specific.
Add note to translators.
Note to translators
Add a missing space and remove a extra.
Thanks to our Norwegian Bokmål translator Allan Nordhøy for finding and
reporting this!
Enhance the "View Certificate" functionality to show issuer information. Refs #7034

--HG--
branch : release-2.x.y
Clarify docs - the PurpleCertificate from purple_certificate_pool_retrieve must
be freed using purple_certificate_destroy.

--HG--
branch : release-2.x.y
s/reason/reasonable/ (thanks, rlaager)
And improve the wording slightly.
Fix Yahoo login when using the GnuTLS library for TLS connections.

I changed our gnutls plugin to always treat a closed TCP connection
as if the remote server cleanly terminated the SSL session. I think
it's sad that this is necessary, but I also think it's fairly
harmless. See my comments in the code for details.

--HG--
branch : release-2.x.y
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.
Whitespace-only change.

I'm about to change this code and I want to make this indentation
change first. I prefer two-tab indent on wrapped lines. Massive
numbers of spaces to try to align code is crazy.
Copy changes from datallah's 2948449ffd12 from the
release-2.x.y branch to default.

I dropped these changes when I merged that branch into default because
the code had moved and it didn't seem appropriate to re-do the change
in the merge commit.

The only remaining change from that commit is to add a "View Issuer
Certificate" button to the accept dialog. I think it's a good idea to
give users a way to view info about the cert chain, but I'm not going
to spend the time to figure out how to do it in default. If anyone else
wants to, feel free.

If anyone is keeping a list of starter tasks for aspiring patch writers,
this is a great one.

Also, why the hell is the x509_display_string() function in the
library-specific code? The function is duplicated and almost 100% identical
in ssl-gnutls.c and ssl-nss.c. Ugh.

Refs #7034
https://developer.pidgin.im/ticket/7034
Move x509_display_string() from ssl-nss.c and ssl-gnutls.c to certificate.c.

We should try to keep the library-specific abstraction files as small
as possible and avoid putting display logic there. The larger the files,
the more likely it is that behavior will differ, and that's bad.