XFCE comes supplied with an xinitrc script, use that instead.
It turns out that making ~/.xinitrc & ~/.xsession a shell script with a bang
path & marked executable is a relic of the past. These files are execed by
the scripts which call them.

Tested on FreeBSD-CURRENT with rene@
PR: 222844
Submitted by: Olivier Duchateau <duchateau.olivier at gmail>
Approved by: bcr (mentor) rene
Differential Revision: https://reviews.freebsd.org/D16687