Keyserver: Made it quit when pressing Ctrl+C, also serve a key off disk

Now, when testing GNOME Keysigning, you don't need to have two instances running.
You can start the keyserver with an exported key.
Merge branch 'keyserver_quit' into tobi_tmp_email
Sections: Introduced a UIDExport function to export a single UID

This is very likely needed for stripping everything but one UID.
Sections: Mailing the signed UID and the UID only

It requires a bit of a dance with GnuPG in order to strip off all other
but the signed UID, but it seems to work well enough for now.