enable keepalive when the keepalive EDNS option is seen

enable keepalive when the keepalive EDNS option is seen

previously, receiving a keepalive option had no effect on how
long named would keep the connection open; there was a place to
configure the keepalive timeout but it was never used. this commit
corrects that.

this also fixes an error in isc__nm_{tcp,tls}dns_keepalive()
in which the sense of a REQUIRE test was reversed; previously this
error had not been noticed because the functions were not being
used.