When embedding JPEGs into a container file format, it may be desirable
to minimize JPEG metadata, since the container will include the pertinent
details.  By default, libjpeg emits a JFIF APP0 segment for JFIF-
compatible colorspaces (grayscale or YCbCr) and Adobe APP14 otherwise.
Add a no_default_app_segments option to disable these.

660894c added code to force emission of the JFIF segment if the DPI is
specified, even for JFIF-incompatible colorspaces.  This seems
inconsistent with the JFIF spec, but apparently other software does it
too.  no_default_app_segments does not disable this behavior, since it
only happens when the application explicitly specifies the DPI.