Convert ls(1) to not use libxo(3)

libxo imposes a large burden on system utilities. In the case of ls, that
burden is difficult to justify -- any language that can interact with json
output can use readdir(3) and stat(2).

Logically, this reverts r291607, r285857, r285803, r285734, r285425,
r284494, r284489, r284252, and r284198.

Kyua tests continue to pass (libxo integration was entirely untested).

Reported by: many
Reviewed by: imp
Discussed with: manu, bdrewery
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13959