Implement and use a generic auth. cookie initialization function.

Implement and use a generic auth. cookie initialization function.

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