More thoughts on emulation

[Zefram <A.Main@xxxxxxxxxxxxxxxxx>]

It seems that we want to treat the emulation mode as a special type of
option.  When I added the settable emulation mode, its only purpose
after initialisation was to select which set of option letters would be
used; if my FUNCTION_ARGZERO and SH_FILE_SUBST patches are applied,
that's still the case.  But as we only have two sets of option letters,
and that isn't likely to change in the forseeable future, it might as
well be controlled by a(nother) normal option.  This would immediately
make it subject to LOCAL_OPTIONS, as well as all the existing option
setting/testing mechanisms.  Then emulate could go back to having its
original one-time effect of merely setting options -- including this
new one.  What do you think?

-zefram