Avoid compilation warnings in nt/*.c files

Avoid compilation warnings in nt/*.c files

* nt/cmdproxy.c (fail, vfprintf, fprintf, printf, warn)
(console_event_handler): Add prototypes.
(canon_filename, skip_space, skip_nonspace, get_next_token)
(batch_file_p, search_dir, make_absolute, try_dequote_cmdline)
(spawn, get_env_size): Now static.
* nt/ddeclient.c (DdeCallback): Provide prototype.
* nt/addpm.c (DdeCallback): Provide prototype.
(add_registry): Now static.