Add internal logging functions to the netmgr

Add internal logging functions isc__netmgr_log, isc__nmsocket_log(), and isc__nmhandle_log() that can be used to add logging messages to the netmgr, and change all direct use of isc_log_write() to use those logging functions to properly prefix them with netmgr, nmsocket and nmsocket+nmhandle.