chroot.2: Update errors and unprivileged use

Include errors common to chroot and fchroot in a single list, followed
by errors unique to each.

Unprivileged chroot is permitted if the security.bsd.unprivileged_chroot
sysctl is set to 1. Make note of this and update the EPERM description.

Reported by: kevans
Reviewed by: kevans, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51703