add isc_nmhandle_settimeout() function

add isc_nmhandle_settimeout() function

this function sets the read timeout for the socket associated
with a netmgr handle and, if the timer is running, resets it.
for TCPDNS sockets it also sets the read timeout and resets the
timer on the outer TCP socket.

(cherry picked from commit 4be63c5b00700f83a96da67cc092bb3b87efaf15)