more possible dead ports cleanup

I think a few more things could removed/simplified after the recent
round of port removal:

- Remove definition of offsetof() in c.h

- (Side point, the definition of endof() in the same place isn't used
anywhere, and doesn't look safe to me, because it can go one past the
end of memory.)

- Remove strtol, strtoul, strdup from libpgport. I seem to recall these
were for nextstep at some point. Before we applied libpgport
everywhere, we were actually already going fine without these in some
parts of the source tree.

Some more risky things that can only be tested at run time;

- We could use fflush(NULL) in src/backend/postmaster/fork_process.c.

- There is some business about realloc(NULL) not working in
src/interfaces/libpq/fe-exec.c.

I suggest we do the first 2 (or 3) now, and leave the last 2 for future
cleanups.