gpg: New option --override-session-key-fd.

* g10/gpg.c (oOverrideSessionKeyFD): New.
(opts): Add option --override-session-key-fd.
(main): Handle that option.
(read_sessionkey_from_fd): New.

The override-session-key feature was designed to mitigate the effect
of the British RIP act by allowing to keep the private key private and
hand out only a session key. For that use case the leaking of the
session key would not be a problem. However there are other use
cases, for example fast re-decryption after an initial decryption,
which would benefit from concealing the session key from other users.

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