Fix mesecon control of forcefield

Fix mesecon control of forcefield

The code formerly attempted to make the forcefield emitter controlled
both manually and by (inverted) mesecon signal, but the two interfered
with each other.  In particular, a newly-placed emitted would be
informed that it was getting no mesecon signal, and would therefore
enable itself.  Fix this by adding explicit modes for how the emitter
will respond to mesecon signals: ignore them, obey them positively,
or obey them negatively.

The manual control could have been incorporated into this mode setting
by having two "ignore mesecon" modes: always-enabled and always-disabled.
But it seems more useful to have a separate manual master switch, so that
the emitter can be manually disabled without losing the mesecon mode.
So it is now implemented that way.