domains: allow pre-defined domains to be unloaded

Add domain_remove() SYSUNINT callback that removes the domain
from the domain list. This change is required to support Netlink.
This version is different from HEAD: it uses fixed family check,
instead of the DOMF_UNLOADABLE flag. The dom_flag field appeared
in HEAD and was not merged back, as there are no spare fields in
'struct domain'.

Original commit message:

Add domain_remove() SYSUNINT callback that removes the domain
from the domain list if it has DOMF_UNLOADABLE flag set.
This change is required to support netlink ( D36002 ).

Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D36173

(cherry picked from commit 9b967bd65de6647aed68a141dc34f9b223a2593c)