We return a pointer when we are using cap_init() or cap_service_open()

We return a pointer when we are using cap_init() or cap_service_open()
function, so check if cap_chanel_t is NULL is not enough.
Casper with a normal libc will still fail in capability mote so let's not
enter capability mode without casper support when we need to resolve DNS.

Reviewed by:	cem
Differential Revision:	https://reviews.freebsd.org/D12823