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