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Also modifiy Makefiles to install perfdhcp into sbin.

 - hooks documentation updated
 - dhcp4 message logs updated
 - long lines wrapped in dhcp4_srv.cc
 - unit-test names now use camel notation
 - Pkt4::bufferOut_ renamed to buffer_out_

Some of the pktN protected variables are still accessed directly in derived
classes thus warnings have been added in libs.