1) New environment variable SLIC3R_PP_HOST contains one of
"File", "PrusaLink", "Repetier", "SL1Host", "OctoPrint", "FlashAir", "Duet", "AstroBox" ...
2) New environment variable SLIC3R_PP_OUTPUT_NAME contains the name
of the G-code file including path (for SLIC3R_PP_HOST == "File")
or a name of the file after upload to the host (PrusaLink, Octoprint ...)
3) The post-processing script may suggest a new output file name
(likely based on SLIC3R_PP_OUTPUT_NAME) by saving it as a single line
into a new "output name" temp file. The "output name" file name is
created by suffixing the input G-code file name with ".output_name".
Please note that the G-code viewer visualizes G-code before post-processing.
Fixes Broken PostProcessing when script changes out-filename #6042
