Increase bufsize and linebuf size for broken MUAs

* src/mimedataprovider.cpp (BUFSIZE, LINEBUFSIZE): Increase.

RFC-5322 Section 2.1.1. clearly states line lenght limits.
But some MUA's, in this case SecurePIM ignore this and
put the full HTML Mail in a single line.

• GnuPG-Bug-Id: T4552