client: enhance the PATH when searching for the 'bup' binary.

client: enhance the PATH when searching for the 'bup' binary.

Automatically adds the *local* $PWD to the *remote* $PATH before trying to
run 'bup server'.  That way, if you build the source in exactly the same
folder on two machines - or if those two machines are actually the same
machine and you're just doing a test against localhost - it'll work.

I hereby curse both "sh -c <command>" and "ssh hostname -- <command>" for
not allowing a sensible way to just set argv[] without doing any stupid
quoting.  Nasty.