copy of the code formerly in mail500, to properly track changes.  An
exception is mail500.m4 that has been committed as maildap.m4 directly
to avoid breaking history twice.

to put it in line with current practices.  Most likely I got them wrong,
so maight not even compile right now.

Back-shell still doesn't support binary data, should use -lldif routines

 (on Linux), with max_concurrency set to 0 (as many as needed), 1, 2,
and 3.

in attempt to get it work with GNU PTH.

macros LUTIL_STRDUP and friends).

l is already parsed?