add "rgbjpeg" flag to tiffsave

add a secret "rgbjpeg" flag to tiffsave to help IIPImage server

iipimage server 0.9.9 and earlier don't handle YCbCr-coded JPEG tiffs
correctly ... setting --rgbjpeg makes vips output RGB coded JPEG tiff
which iipsrv can handle.

Using this flag makes files 2x to 3x larger with no quality improvement,
so it is tagged as deprecated to keep it hidden.
fix a warning
Merge remote-tracking branch 'origin/7.38'