lib: Added i_stream_create_min_sized*()

lib: Added i_stream_create_sized_range() This is just a small wrapper to i_stream_create_sized() to specify stream's start offset.

lib: istream-sized - set stream_errno=EPIPE if stream is too small

lib: Added i_stream_create_sized_with_callback(). The callback returns the wanted error string.

istreams: When reading invalid data, fail with EINVAL instead of EIO. EINVAL was already used more.

Renamed istream-attachment to istream-sized. The code didn't really assume anything about the input being an attachment, so this allows reusing the stream for other purposes.