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 - flag => configuration parameter
 - echoClientId test uses ASSERTs rather than EXPECTs

 - tests/methods descriptions improved
 - CfgMgrTest.echoClientId improved
 - Redundant checks removed from Dhcpv4SrvTest.discoverEchoClientId
 - BIND10 Guide clarified
 - ChangeLog added

This commit includes trivial changes to the doxygen documentation and
make one variable const.

trac3186 added the user_chk hook library which had a couple of
uses of EXPECT_EQ on stl containers. Gtest 1.5 does not like this.

After merging, distcheck revealed a few minor issues with logging
files and unit test data files. These were corrected.

Adds the hooks subdirectory to bind10/dir for storing hooks shared
libraries, and the first such library for DHCP, user_chk.

Mostly minor cosmetics. Added logic to select callouts to
simply return if no subnets are configured.

Conflicts:
	ChangeLog

Conflicts:
	ChangeLog

 - Guide updated
 - Iface::addUnicast() now refeses duplicates
 - addUnicast and clearUnicasts are now commented
 - new unit-tests
 - argument for addActiveIface is const reference again
 -

 - default value is no longer 0.0.0.0, so global value is no longer
   overwritten with subnet-specific values
 - added 2 new unit-tests to check if the value is really set in
   server responses
 - guide updated to explain that
 - fix in Dhcp4ParserTest.nextServerNegative

Added unit test data file, overlooked with prior commit. Also
added missiong include to user_file.cc and took extraneous debug
cout.

Also, clear the output buffer in the Pkt4 and Pkt6 every time when pack()
is called.

Conflicts:
	src/bin/dhcp4/dhcp4_srv.cc
	src/lib/dhcp/libdhcp++.cc

Added doxygen throughout, cleaned up unit tests.  Removed CLIENT_ID type
as it was unnecessary.