gpg: Reject signatures made with MD5.

* g10/gpg.c: Add option --allow-weak-digest-algos.
(main): Set option also in PGP2 mode.
* g10/options.h (struct opt): Add flags.allow_weak_digest_algos.
* g10/sig-check.c (do_check): Reject MD5 signatures.
* tests/openpgp/gpg.conf.tmpl: Add allow_weak_digest_algos.
--
(cherry picked from commit f90cfe6b66)

Resolved conflicts:
	g10/gpg.c - adjust.
	tests/openpgp/defs.inc - no changes

doc/gpg.texi

@@ -2178,7 +2178,7 @@ available, but the MIT release is a good common baseline.
 
 This option implies
 @option{--rfc1991 --disable-mdc --no-force-v4-certs
- --escape-from-lines  --force-v3-sigs
+ --escape-from-lines  --force-v3-sigs --allow-weak-digest-algos
  --cipher-algo IDEA --digest-algo MD5 --compress-algo ZIP}.
 It also disables @option{--textmode} when encrypting.
 
@@ -2608,6 +2608,13 @@ necessary to get as much data as possible out of the corrupt message.
 However, be aware that a MDC protection failure may also mean that the
 message was tampered with intentionally by an attacker.
 
+@item --allow-weak-digest-algos
+@opindex allow-weak-digest-algos
+Signatures made with the broken MD5 algorithm are normally rejected
+with an ``invalid digest algorithm'' message.  This option allows the
+verification of signatures made with such weak algorithms.
+
+
 @item --no-default-keyring
 @opindex no-default-keyring
 Do not add the default keyrings to the list of keyrings. Note that