This cuts out some duplicate code, and enables us to reuse this
logic for PNG, JPEG, and any other future output format.
sanei_load_icc_profile also allows us to know that an ICC profile
file is not long enough before we start to write it to the output;
this should prevent poorly-written software from overflowing into
image data when they read the bad profile based on its length.
* frontend/scanimage.c, frontend/stiff.h, frontend/stiff.c:
Added option "-i filename"/"--icc-profile filename":
When image is saved in tiff format then this icc-profile is
added to the tiff file as tiff tag