gpg: Distinguish between missing and cleared key flags.

* include/cipher.h (PUBKEY_USAGE_NONE): New.
* g10/getkey.c (parse_key_usage): Set new flag.

We do not want to use the default capabilities (derived from the
algorithm) if any key flags are given in a signature. Thus if key
flags are used in any way, the default key capabilities are never
used.

This allows to create a key with key flags set to all zero so it can't
be used. This better reflects common sense.
(cherry picked from commit 4bde12206c5bf199dc6e12a74af8da4558ba41bf)
(cherry picked from commit 0a805ed1604ef3e9b27f3e22a936a2d439300e9f)

Resolved conflicts:
include/cipher.h