write_idxfile(): New function. Isolate format of the external index

write_idxfile():  New function.  Isolate format of the external index
	data file used by buildindex.py.

next_argument_id():  Removed; no longer used.

next_argument():  Doesn't use next_argument_id(), just does the work.

gen_target(), gen_target_name():  Removed, no longer used.  The only
	place that used them now does the (trivial) work directly.

Lots of little changes so we never have to pass around $br_id values
all over the place.  The only places they're used now new IDs are
generated as needed.