mhbuild and long header fields

[Philipp]

Hi

I have noticed that mhbuild don't implement header field folding.
Specialy for the References field this might cause problems, when you
use it to feed a mail build by mhbuild to a tool which checks the
line length.

Also I don't get the code. In uip/mhbuildsbr.c:359 a LENERR is handled
with die(), but I can't create a testcase for this.

Philipp