Adjust the semantics of cfg.remote_cmd_ptr: it is now NULL when

cfg.remote_cmd is to be used, rather than actually pointing at
cfg.remote_cmd. This change restores the ability to structure-copy
Configs without breaking them. (Though of course this is only a
temporary solution: really what wants doing is to fix
`config-struct'.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@5335 cda61777-01e9-0310-a592-d414129be87e