New proc for config/unix.exp

[Ben Elliston <bje at redhat dot com>]

I would like to propose a new proc for the test harness to relieve the
tedium of a common idiom:

  # assert that a bus transaction was "ok"
  proc sid_assert_bus_ok {result} {
       return ![string compare [lindex $result 0] ok]
  }

I assume no one has any objections in principle?

Ben