Reset signal and exit handlers in subshells.

* geesh/shell.scm (%subshell): In the child process, restore all
signal handlers to their original actions and unset the current
'atexit' handler.

geesh/shell.scm

@@ -223,6 +223,8 @@ process."
   (match (primitive-fork)
     (0 (with-continuation-barrier
         (lambda ()
+          (restore-signals)
+          (set-atexit! #f)
           (thunk)
           (primitive-exit (get-status))))
        (primitive-exit 1))