Fix a couple of issues with our key parsing / validating code.

- be explicit and expect only ssh-dss or ssh-rsa keys
- allow any character (even line breaks and whitespace) in the optional comment,
  but shrink all of them to simple spaces (fixes issue 679)
- test the newly uploaded key against existing keys only by the base key data,
  not the fully uploaded string (that might contain a changed comment line or
  the like) to avoid duplicates; also only check the keys of the user for
  duplicates, not all existing keys in the forge (if for whatever reason two
  user accounts share a key)

NEWS.mdtext

@@ -7,6 +7,7 @@
 - monotone zip archive entries now all carry the revision date as mtime (issue 645)
 - Timeline only displays filter options for items a user has actually access to (issue 655)
 - Fix the self-link of the RSS feed (issue 666)
+- Fix SSH public key parsing issues and improve the check for existing, uploaded keys (issue 679)
 
 ## Documentation