Shrink encoded line length from 76 to 72 characters.

Shrink encoded line length from 76 to 72 characters.

Some Exchange systems wrap lines over 75 characters long while converting
messages to quoted-printable, preventing ctm_rmail from reassembling
emailed deltas.  For a negligible loss of encoding efficiency, this change
allows ctm deltas to once more pass through Exchange undamaged.