jail: Add the ability to access system-level filesystem extended attributes

Prior to this commit privileged accounts in a jail could not access to the
filesystem extended attributes in the system namespace. To control access to
the system namespace in a per-jail basis add a new configuration parameter
allow.extattr which is off by default.

Reported by: zirias
Tested by: zirias
Obtained from: HardenedBSD
Reviewed by: kevans, jamie
Differential revision: https://reviews.freebsd.org/D41643
MFC after: 1 week
Relnotes: yes