gpg-interface: use more status letters

gpg-interface: use more status letters

According to gpg2's doc/DETAILS:

    For each signature only one of the codes GOODSIG, BADSIG,
    EXPSIG, EXPKEYSIG, REVKEYSIG or ERRSIG will be emitted.

gpg1 ("classic") behaves the same (although doc/DETAILS differs).

Currently, we parse gpg's status output for GOODSIG, BADSIG and
trust information and translate that into status codes G, B, U, N
for the %G?  format specifier.

git-verify-* returns success in the GOODSIG case only. This is
somewhat in disagreement with gpg, which considers the first 5 of
the 6 above as VALIDSIG, but we err on the very safe side.

Introduce additional status codes E, X, Y, R for ERRSIG, EXPSIG,
EXPKEYSIG, and REVKEYSIG so that a user of %G? gets more information
about the absence of a 'G' on first glance.

Requested-by: Alex <agrambot@gmail.com>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>