[RFC] Add hash token to ControlPath

[mancha]

mancha <mancha1 <at> hush.com> writes: 
> Attached patch adds a new percent-token (%H) that expands to the
> sha1 digest of the concatenation of host (%h) + port (%p) + remote
> user (%r). The token's expanded length is a fixed 40 characters
> and, barring digest collision, provides uniqueness.
> 
> The patch was built against 6.5p1 but applies (with harmless
> offsets) to OpenBSD HEAD.
> 
> --mancha

Apologies but it seems the ML strips attachments. Rather than risk
whitespace debacles with inline patches, I've placed it here:

http://sf.net/projects/mancha/files/misc/openssh-6.5p1-mux-hash.diff