This used to be a configuration option because spectator mode and "net"
mode were incompatible. When the ability to switch between player and
spectator was added, the configuration option was removed, since it was
no longer a mode toggle. This re-adds it, mainly so that I can use it to
implement regression tests.
This patch transfers core_reset across netplay. Resets effectively
worked before thanks to check_frames, but this makes resets work even
without check_frames, and in particular should allow resets to force
sync in savestateless cores, bringing them one step closer to actually
being usable by non-experts.
Previously, if two clients were connected to the same server and one of
them was ahead of the server, the only way to rectify that situation was
for the client to get so far ahead that it stalled, as the server could
only catch up with an ahead client if all clients were ahead. That's
unrealistic. This gives the server the alternate option of demanding
that a client stall. This keeps things nicely in line even with >2
players.
Making the netplay handshake protocol send the core and content as an
explicit command, so that the other side can (notionally) choose to load
it. That isn't implemented, of course.
