gpg: Add "self-sigs-only" and "import-clean" to the keyserver options.

* g10/gpg.c (main): Change default.

Due to the DoS attack on the keyeservers we do not anymore default to
import key signatures. That makes the keyserver unsuable for getting
keys for the WoT but it still allows to retriev keys - even if that
takes long to download the large keyblocks.

To revert to the old behavior add

keyserver-optiions  no-self-sigs-only,no-import-clean

to gpg.conf.

• GnuPG-bug-id: T4607
• Signed-off-by: Werner Koch <wk@gnupg.org>