libjpeg-turbo diff review

These patches encompass various suggested changes, based on a thorough diff review of libjpeg-turbo and mozjpeg.  They include some fixes for incorrect merges (vestigial code and whitespace formatting left over from libjpeg-turbo 1.3.x that shouldn't be there anymore), fixes for incorrect global search/replace of the word "turbo" in the documentation (for instance, the build instructions were incorrectly referring to a "MozJPEG" API-- that isn't actually a thing), mozjpeg-specific descriptions and package names to avoid conflict or confusion (if someone actually does decide to use the inbuilt packaging mechanism), and changing the awkward "libmozjpeg" designation to "mozjpeg".

It may make sense to also remove the build instructions for mobile devices, since those are mainly focused on decoding and mozjpeg is for encoding only.