Split main.c into main.c and mainloop.c

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.

src/test/test_mainloop.c

@@ -10,7 +10,7 @@
 #include "test/log_test_helpers.h"
 
 #include "core/or/or.h"
-#include "core/mainloop/main.h"
+#include "core/mainloop/mainloop.h"
 
 static const uint64_t BILLION = 1000000000;