Enable the load-balance-sockets configuration
Previously, HAVE_SO_REUSEPORT_LB has been defined only in the private netmgr-int.h header file, making the configuration of load balanced sockets inoperable.
Move the missing HAVE_SO_REUSEPORT_LB define the isc/netmgr.h and add missing isc_nm_getloadbalancesockets() implementation.
Closes #3249 (closed)