Rsync backend: allow remotes

this should allow all ssh remotes permitted by rsync. Still dubbed 'local' backend
in the code, and needs testing

use e.g. `user@host:dir/to/project/`. Don't forget the trailing slash.
Merge remote-tracking branch 'OCamlPro/allow-ssh'

Conflicts:
	src/client/opamPinCommand.ml
	src/core/opamTypesBase.ml
	src/repositories/opamLocal.ml