Update mailing list URLs

- SourceForge dropped the archive for the inactive squirrelmail-lang-es
- Carnegie Mellon University dropped the legacy archive for
  squirrelmail-imapproxy
- MARC dropped the archive for squirrelmail-imapproxy
- Nabble has changed all URLs
- Nabble also have a list named squirrelmail-siteupdates but that's
  intentionally left out since no such list exists

Some whitespace changes are there as well, so diff with -w for a more
pleasant diffing experience.