gpg: Change --show-session-key to print the session key earlier.

* g10/cpr.c (write_status_strings): New.
(write_status_text): Replace code by a call to write_status_strings.
* g10/mainproc.c (proc_encrypted): Remove show_session_key code.
* g10/decrypt-data.c (decrypt_data): Add new show_session_key code.
--

This feature can be used to return the session key for just a part of
a file.  For example to downloading just the first 32k of a huge file,
decrypting that incomplete part and while ignoring all the errors
break out the session key.  The session key may then be used on the
server to decrypt the entire file without the need to have the private
key on the server.

GnuPG-bug-id: 1389
Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit 101a54add351ff62793cbfbf3877787c4791f833)

Resolved Conflicts:
	doc/DETAILS    - removed
	g10/cpr.c      - replace estream fucntion by stdio.
	g10/mainproc.c - Adjust for changed calling convention.