Use the generic function for both the ControlPort cookie and the
ExtORPort cookie.
Also, place the global cookie variables in the heap so that we can
pass them around more easily as pointers.
Also also, fix the unit tests that broke by this change.
Conflicts:
src/or/config.h
src/or/ext_orport.c
Incidentally, this business here where I make crypto_rand mockable:
this is exactly the kind of thing that would make me never want to
include test-support stuff in production builds.
