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.