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Conflicts:
	src/lib/dhcp/iface_mgr.cc

The DHCPv4 server is now responsible for setting up a correct unicast
source address for the outgoing packet. The IfaceMgr doesn't replace
the broadcast address with the unicast address anymore.

   Completes initial implmementation of example hooks libary, user_chk.

Removed reponse packet type check from IPv6 packet_snd callout.
Corrected text in user chk doxygen page.

Addressed review comments which were largely minor. Limited use of
extern C linkage to only the callout functions themselves. Added
a dox page describing the library.  Added namespace user_chk.

3207 was created quite some time ago, so master was
merged into it first.

The obsolete usage of AC_OUTPUT was actually causing config.status
to be executed multiple times, which is a big waste of time. Changed
to use the correct AC_CONFIG_FILES and AC_CONFIG_COMMANDS macros
instead.

Conflicts:
	src/bin/d2/d2_messages.mes