Create the long-awaited console.c, and move the common routines out

Create the long-awaited console.c, and move the common routines out
of scp.c, psftp.c and plink.c into it. Additionally, add `batch
mode', in which all the interactive prompts (bad host key, log file
exists, insecure cipher, password prompt) are disabled and safe
responses are assumed. (The idea being that if you run PSCP, for
example, in a cron job then you'd probably rather it failed and
exited instead of leaving the cron job wedged while it waits for
user input that will never arrive.)

[originally from svn r1525]