gpg: New import and keyserver option "self-sigs-only"

* g10/options.h (IMPORT_SELF_SIGS_ONLY): New.
* g10/import.c (parse_import_options): Add option "self-sigs-only".
(read_block): Handle that option.

This option is intended to help against importing keys with many bogus
key-signatures. It has obvious drawbacks and is not a bullet-proof
solution because a self-signature can also be faked and would be
detected only later.

• GnuPG-bug-id: T4591
• Signed-off-by: Werner Koch <wk@gnupg.org>