construct/librb

This is libircd from ircd-ratbox.  A few notes about this library:

1. Most of this code isn't anywhere near threadsafe at this point.  Don't
   hold your breath on this either.
2. The linebuf code is designed to deal with pretty much 512 bytes per line
   and that is it.  Anything beyond that length unless in raw mode, gets
   discard.  For some non-irc purposes, this can be a problem, but for
   ircd stuff its fine.
3. The helper code when transmitting data between helpers, the same 512 byte
   limit applies there as we recycle the linebuf code for this.