[2982] First draft of Hooks user guide
[2995] 4 hook points implemented:

 - pkt6_receive, subnet6_select, lease6_select, pkt6_send
 - framework updated
 - some unittests implemented
[2995] pkt6_receive callout is now fully tested.
[2995] Default hook indexes are now -1
[2995] Four unit-tests for pkt6_send implemented.
[2995] First unit-test for subnet6_select hook implemented.
[2995] Remaining test for subnet6_select implemented.
[2995] Initial implementation of lease6_select hook (segfaults for now)
[2771] convert HTML-style tables to docbook

NOTE: that my renderer, dblatex, is putting the huge table on one page.
I need to solve that.
[2995] lease6_select unit-tests are now working.
[2995] Minor tweaks in lease6_select.
[2995] Dev guide extended to cover Hooks API for DHCPv6
[2995] Dhcpv4Srv tests are now passing again.
[2995] Some editing of the documentation.
[2856] Extend SegmentInfo and add some basic tests

This is according to the description in #2856, but it needs further
testing and also documentation. These will be added in future commits
after handling "load" command in the MemorySegmentBuilder.
[2856] Move command handlers into their own methods
[2856] Pass tuples in the command queue
[2856] Add "load" command handler
[2995] First set of changes after review.
[2995] Merge branch 'trac2980' into trac2995

 - Also code updated to match changes from 2980

Conflicts:
	src/lib/hooks/callout_handle.h