Make comment about net-snmp 5.4.x workaround more verbose
Abstract certain functions from actual library for compatibility with net-snmp <5.6

From net-snmp 5.5 to 5.6, all code from libnetsnmphelpers.so got integrated into libnetsnmpagent.so. While this doesn't break C programs since ld-linux.so will take care of dynamically resolving referenced symbols, with ctypes all prototype definitions and functions calls always depend on a particular libX handle.

Therefore we now test using netsnmp_create_watcher_info whether that function is available in libnetsnmpagent and fall back to libnetsnmphelpers otherwise. The remaining code now uses libnsX to abstract from the actual library used.