[RFC] Add hash token to ControlPath

[mancha]

Hi.

Last night on an irc openssh channel, a user brought up a use
case involving cluster trees and very descriptive (i.e. long)
hierarchical hostnames.

To make a long story short, his ControlPath (~/.ssh/control-master
/%r@%h:%p) was bumping up against UNIX_PATH_MAX.

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