Clarify our #if{n}def by commenting what they are at the #elif/#else/#endif

Ok that's an easy one!

The idea is to improve our comments in the .h/.c files where we have a #ifdef BLAH and the corresponding #endif is at least a non negligible amount of lines after. Of course that goes without saying to also comment the #ifndef. Pattern would look like this:

#ifdef BLAH
[some non negligible amount of lines]
#elif /* BLAH */
[some other amount of lines]
#endif /* BLAH */

According to nickm, the current situation is:

Our headers have 27 #endifs with a comment, and 435 without.

Good example is or.h:

#ifndef TOR_OR_H
[5000+ lines in between]
#endif /* TOR_OR_H */

This file src/or/shared_random.h is a good example of what it could look like.