1.0.25.26: less interrupt handling leftovers

1.0.25.26: less interrupt handling leftovers

- less clear_pseudo_atomic_interrupted

  Remove it from the beginning of the protected sections in the
  runtime to match what we do in Lisp.

- less resetting of signal mask

  Don't call reset-signal-mask from the toplevel, I don't think it's
  useful anymore and quite likely it was never more than duct tape.

  Even when unmasking of signals is called for (in
  INVOKE-INTERRUPTION), only unblock the deferrable ones: gc signals
  are unblocked in Lisp anyway.

- removed unneeded sigemptyset on pending mask

- move MIPS SIGTRAP workaround to the runtime