ssh: provide a factory function for setting ssh paths

git allows you to set which paths to use for the git server programs
when connecting over ssh; and we want to provide something similar.

We do this by providing a factory function which can be set as the
remote's transport callback which will set the given paths upon
creation.
Merge pull request #2463 from libgit2/cmn/ssh-factory-for-paths

ssh: provide a factory function for setting ssh paths