Allow PSFTP to be run with no arguments, in which case it enters the

command-line state but all commands are disallowed except `open
host.name'. The idea is to provide marginal extra niceness for
people who double-click the icon without realising it's a cmdline app.

git-svn-id: svn://svn.tartarus.org/sgt/putty@1480 cda61777-01e9-0310-a592-d414129be87e