Changed m_getfld() buffer size to 4096 from 8192, to benefit

Changed m_getfld() buffer size to 4096 from 8192, to benefit
scan(1).  We could consider using a larger buffer size when
readying message bodies.  But I expect any benefit would be
overshadowed by the way parse_mime() currently reads MIME
messages:  it reopens the file for each part.