It differs from the rest of the rephist code in that it's actually
necessary for Tor to operate, so it should probably go somewhere
else.  I'm not sure where yet, so I'll leave it in the same
directory, but give it its own file.

Make them only include the headers that they needed, and sort their
headers while we're at it.

This is preparation for having a routermode.h as well

Fixes a stem test failure; bugfix on c7ce6b98. Bug
not in any released Tor.

The main.c code is responsible for initialization and shutdown;
the mainloop.c code is responsible for running the main loop of Tor.

Splitting the "generic event loop" part of mainloop.c from the
event-loop-specific part is not done as part of this patch.