upstream: support for using the SFTP protocol for file transfers in

scp, via a new "-M sftp" option. Marked as experimental for now.

Some corner-cases exist, in particular there is no attempt to
provide bug-compatibility with scp's weird "double shell" quoting
rules.

Mostly by Jakub Jelen in GHPR#194 with some tweaks by me. ok markus@
Thanks jmc@ for improving the scp.1 bits.

OpenBSD-Commit-ID: 6ce4c9157ff17b650ace571c9f7793d92874051c
