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

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: 4591
Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit 15a425a1dfe60bd976b17671aa8e3d9aed12e1c0)
(cherry picked from commit adb120e663fc5e78f714976c6e42ae233c1990b0)

Gbp-Pq: Topic from-2.2.17
Gbp-Pq: Name gpg-New-import-and-keyserver-option-self-sigs-only.patch