Clean up Transport::execute() and callers.

- Ensure that the real exit code of the child program is retrieved
  using WEXITSTATUS().

- Centralise the handling of code 127, which means that the child
  shell process could not execute its child (ssh, rsync, or curl.)

Signed-off-by: Russell Steicke <russells@adelie.cx>
Transport Errors

- The 'push' and 'pull' commands now properly distinguish between a missing
  transport utility and other errors (thanks to Russell Steicke).