[FIX] Only store keyservers that actually work [#299 state:fixed]

- Before storing keyserver entered in preferences is stored, it's verified that it works
  and only then the updated keyserver is written to gpg.conf
- Prior to this change, the keyserver was stored regardless whether it worked or not, thus
  antering a keyserver with a new line (\n) character completely broke GPG Keychain.