make READLINK work on osx

osx provides 'readlink' and 'greadlink', the latter is what we want.
so we
- swap tests (first 'greadlink', fall back to 'readlink')
- check whether the command found supports the '-f' flag
add a normalize message to enable / disable output normalisation
improved help file

add new feature and avoid pd hang due to print too much data
use MACRO instead
add RGB and RGBA mode like pix_set

TODO : add RED, GREEN, BLUE and ALPHA mode
rename "normalize" to "bytemode"
reflect last changes (method renaming) in help file
retrieve ROI for all _PIX objects
add ROI support
add a bytemode message to follow pix_dump behavior

this is usefull to send image as OSB blob
remove unneeded lines
initialize inputScale
use 'bool' rather than 'int' for boolean values
use 'bool' callbacks