Use readlink(1)/stat(1) to query symlinks instead of file(1)

file(1) can be compiled out of the system via MK_FILE == no, and the
output isn't guaranteed to be stable. It's better to use stat(1)/readlink(1)
instead to query symlink/file paths.

MFC after: 1 month
MFC with: r319714, r319854
Reported by: ngie
Submitted by: shivansh
Differential Revision: D11159 (part of a larger diff)
Sponsored by: Google, Inc (GSoC 2017)