On OS X 10.15, / is read-only and paths inside of /Users (and elsewhere)
are mounted via a "firmlink" (which is a new invention in APFS). These
must be resolved to their full path to be shareable via NFS.
/Users/johnsmith/mycode => /System/Volumes/Data/Users/johnsmith/mycode
We check to see if a path is mounted here with `df`, and prepend it.
Firmlinks are only createable by the OS and this wasn't supposed to be
visible to applications anyway:
https://developer.apple.com/videos/play/wwdc2019/710/?time=481
Brian Cain
Chris Roberts
Andy Fowler