Apply find_* function refactorings

[clumens]

This is the next batch of refactorings from PR #82, though there are a couple differences from what's in those patches:

• Note the FOREACH_NODE -> _FOREACH_NODE stuff. Basically, I renamed it and added a new version that takes an additional config pointer argument so I don't have to update all callers everywhere at once, along with all their callers, and so on.

• These patches built, but would crash because message_recv has a new parameter, but that function is only ever indirectly called through function pointers, and the use of those pointers did not pass the config pointer. So, various other places had to get that argument added. I did not check very hard to see how this gets handled in the original PR in later patches.

• I added a function pointer type to simplify having to deal with the above.