pubkey: Add forward compatibility feature.

* cipher/pubkey-util.c (_gcry_pk_util_parse_flaglist): Add
"igninvflag".

If future versions of Libgcrypt want to add optional flags to a pubkey
s-expression, they may use the "igninvflag" flag to make the flag
parser ignore flags it does not know about.

• Signed-off-by: Werner Koch <wk@gnupg.org>