Add support for real URI syntax in remote targets

Add support for real URI syntax in remote targets

We still only support SSH for now, but this opens the door for other
protocols.  All of the following should work:

  --remote host
  --remote user@host
  --remote ssh://host/
  --remote ssh://user@host/

URI paths, queries, and fragments are all parsed, but don't have any
meaning yet, so they're treated as an error.  I can imagine using this
for protocol-specific options though, e.g. ssh://host/?Compression=yes

* remote.cxx (class uri_decoder): New, breaks down a URI string.
  (ssh_remote::ssh_remote): Add a constructor using a uri_decoder.
  (remote::create): Redirect based on the scheme of URI-looking remote
  targets.  Assume all others are still plain [user@]host for ssh.