this commit removes isc__nm_tcpdns_keepalive() and isc__nm_tlsdns_keepalive(); keepalive for these protocols and for TCP will now be set directly from isc_nmhandle_keepalive(). protocols that have an underlying TCP socket (i.e., TLS stream and HTTP), now have protocol-specific routines, called by isc_nmhandle_keeaplive(), to set the keepalive value on the underlying socket.