sshaskpass only mode, debug for PromptType.

added --askpass/-a flag to put it in a mode that is a drop-in replacement for x11-ask-pass, that is, do not leak keys in any wallet that may be open. (also aligned the descriptions, the about and git repo description makes it clear it is a wallet glue for ssh-add, but the cli description make it look like a proper ssh-askpass. also put a named position to make clear the 1st param is the prompt message)

more questionably, added --debug/-d. only debug statements are around the logic for SSH_ASKPASS_PROMPT.