gpg: New option --assert-signer.

* g10/gpg.c (enum cmd_and_opt_values): Add oAssertSigner.
(opts): Add "assert-signer".
(main): Set option.
(assert_signer_true): New var.
(g10_exit): Evaluate new var.
* g10/main.h (assert_signer_true): Declare new var.
* common/status.h (STATUS_ASSERT_SIGNER): New.
* g10/options.h (opt): Add field assert_signer_list.
* g10/verify.c (is_fingerprint): New.
(check_assert_signer_list): New.
* g10/mainproc.c (check_sig_and_print): Call that function.  Clear
assert_signer_true on a warning.

* g10/gpgv.c: Add dummy function and vars.
* g10/t-keydb-get-keyblock.c: Ditto.
* g10/t-keydb.c: Ditto.
* g10/t-stutter.c: Ditto.
--

g10/options.h

@@ -235,6 +235,10 @@ struct
      value. */
   int limit_card_insert_tries;
 
+  /* The list of --assert-signer option values.  Note: The values are
+   * modify to be uppercase if they represent a fingerrint */
+  strlist_t assert_signer_list;
+
   struct
   {
     /* If set, require an 0x19 backsig to be present on signatures