agent: Fix binary vs. text mode problem in ssh.

* agent/command-ssh.c (file_to_buffer)
(ssh_handler_request_identities): Open streams in binary mode.
(start_command_handler_ssh): Factor some code out to ..
(setup_ssh_env): new function.

This is for now a theoretical fix because there is no ssh client yet
which uses the GnuPG style IPC. OpenSSL for Cygwin uses only a quite
similar one. gniibe suggested to implement that IPC style in
Libassuan so that a Cygwin version of OpenSSL may be used with GnuPG.

(cherry picked from commit ed056d67c7c93306b68829f83a2565e978dcfd9b)

Also fixed one typo.

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