This adds a new mechanism to say "replace struct S with #include <bits/S.h>".
Also switch epoll_event over to the new mechanism.
Also use the kernel's struct sockaddr_storage directly rather than behind
an unnecessary #define.
This patch also removes some dead code in the header scrubber. This code
still needs rewriting completely. I learned that a "block" isn't necessarily
a single struct definition, say; it might be a run of them. It seems like
a block is a run of preprocessor directives or a run of regular code.
Bug: https://issuetracker.google.com/36987220
Test: new test
Change-Id: Ic6a5c09559766a4babe3cd4c3ea538b885e07308
64f355ffbd