gpg: Print revocation reason for "rvs" records.

* g10/import.c (get_revocation_reason): New.
(list_standalone_revocation): Extend function.
--

Note that this function extends the "rvs" field signature-class (field
11) with the revocation reason.  GPGME does not yet parse this but it
can be expected that the comma delimiter does not break other parsers.

A new field is added to the "rvs" (and in future also the "rev")
record to carry a record specific comment.  Hopefully all parsers
meanwhile learned the lesson from other new fields and don't bail out
on more fields than they know about.

This is partial solution to
GnuPG-bug-id: 1173

Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit b7cd2c2093)

doc/DETAILS

@@ -180,6 +180,9 @@ described here.
     revocation key is also given here, 'x' and 'l' is used the same
     way.  This field if not used for X.509.
 
+    "rev" and "rvs" may be followed by a comma and a 2 digit hexnumber
+    with the revocation reason.
+
 *** Field 12 - Key capabilities
 
     The defined capabilities are:
@@ -260,6 +263,12 @@ described here.
     optionally followed by a space and an URL.  This goes along with
     the previous field.  The URL is quoted in C style.
 
+*** Field 21 - Comment
+
+    This is currently only used in "rev" and "rvs" records to carry
+    the the comment field of the recocation reason.  The value is
+    quoted in C style.
+
 ** Special fields
 
 *** PKD - Public key data