doc/guide/logging.xml - added new logger description
src/bin/dhcp4/dhcp4_log.cc - added bad_packet logger and log name declarations
src/bin/dhcp4/dhcp4_log.h - added bad_packet and log name externs
src/bin/dhcp4/dhcp4_messages.mes - changed drop and NAK message identifiers
src/bin/dhcp4/dhcp4_srv.cc - changed packet drop and NAK log invocations
src/bin/dhcp4/main.cc - used extern for log name