split daemon/bindings.c file

It's now a directory, one C file for each lua table of functions.
We get more total lines due to per-file copyright headers,
but the original file was just too long (nearing 2k lines).
The layout was inspired by the lib/cache/ split.

Disadvantage: git operations (e.g. blame) will stumble on this commit.

During the move, rename the "str" and "xstr" macros, too.
Otherwise there are no real changes, as they would be hard to spot.