Allow dhclient and ping to build WITHOUT_DYNAMICROOT

Allow dhclient and ping to build WITHOUT_DYNAMICROOT

dhclient and ping normally use libcasper services.  These are not
available in statically-linked binaries, so when WITHOUT_DYNAMICROOT is
set disable libcasper use, as with rescue builds.  Also emit a warning
as it's undesirable to build this way.

Reported by:	Michael Dexter
Reviewed by:	rgrimes
Tested by:	Michael Dexter
Approved by:	re (kib)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D17074