Refactor OS detection
Refactor OS detection

Darwin is merged with FreeBSD as all Darwin implementation are already
delegating to FreeBSD implementations.

dan-cleinmark/liquidprompt,jonathanjsimon/liquidprompt,dtschan/liquidprompt,smadhueagle/liquidprompt,lyoshenka/liquidprompt,joshuarubin/dotfiles,nojhan/liquidprompt,pkkolos/liquidprompt,joshuarubin/dotfiles,illwieckz/liquidprompt,tstone2077/liquidprompt,joshuarubin/dotfiles,jaesivsm/liquidprompt,enrdstrm/liquidprompt,BobHood/liquidprompt,elementalvoid/liquidprompt,milouse/liquidprompt,laurentbardin/liquidprompt,dpilone/liquidprompt,tocomo/liquidprompt,colinlieberman/liquidprompt,timhyson/liquidprompt,joshuarubin/dotfiles,kenrachynski/liquidprompt,alexprengere/liquidprompt,tsbarnes/liquidprompt,tkuminecz/liquidprompt,mathieumd/liquidprompt