Update tty command completion to ignore #shell and #suspend when
they're disabled. (Since they aren't flagged for command completion,
this should be unnoticeable.)
Update X11 extended command selection to not show shell and suspend
in the menu when they're disabled. (Trickier than I expected.)
X11 currently rejects #suspend (at run time, not compile time) but
allows #shell. If it was launched syncronously from a terminal
window, shell escape behaves sanely. Otherwise, that seems like
asking for trouble.