BZR_SSH should allow setting the path to ssh, not just the kind of ssh

[Martin Pool]

In 1.0 BZR_SSH can be "paramiko", "plink", "ssh" etc to say what kind of ssh client you want to use. However, there's no apparent way to give the command or path to use.

This is a problem because
1- it's inconsistent with cvs, rsync, etc use of *_SSH
2- some people may not want to use the implementation on their path and want to specify the full path

However, we do need to also know what vendor it is, to know what argument syntax to use amongst other things. The command name alone is probably not enough because openssh and f-secure ssh are both called "ssh" but have different option meanings. So we might need BZR_SSH_VENDOR.

It might be useful if these were also available as configuration variables.

Also, the variable is barely documented, and should be properly explained.

[Martin Pool (mbp)]

Patch needs review: http://bundlebuggy.aaronbentley.com/project/bzr/request/%3Cd80792120908062317h7a283441pb2ecef262fbb0ac3%40mail.gmail.com%3E

[Martin Packman (gz)]

Merged in r4809