daemon(8): when -u option is used set USER, HOME and SHELL variables.

This is consistent with what other uid-morphing utilities
do, i.e. jexec(1), su(1) etc.

Reviewed by: gbe
Differential Revision: https://reviews.freebsd.org/D36148

(cherry picked from commit 6b3ad1d737321cf6a70e957fb15d5287c378373c)