Note: This change fulfills both 2699 (lease4) and 2703 (lease6).

Conflicts:
	src/lib/dhcpsrv/alloc_engine.cc

The selectSubnet() function offers two ways to find a subnet for a
particular interface: using the name of the interface that the subnet is
configured for; and using the packet source address. Also, the redundant
warning is removed if suitable subnet was not found as this warning is
already issued elsewhere.

Conflicts:
	ChangeLog

Conflicts:
	ChangeLog

Conflicts:
	src/lib/python/isc/ddns/session.py

ignoring the result fixes a scan-build error.  besides, ignore() should be
more appropriate choice for what it intends to do anyway.

without [] the output it seems possible that the output is empty for some
environments.  also clarified that we can ignore the cases where -L isn't
contained at all.

Also explicitly added a note in the "Installation" section referring
to the BIND 10 DHCP database installation and configuration section.

Explicit allocation failures cause the return of packet to the client
indicating that the address allocation has been refused.  Also added
was a "catch" for other exceptions - this causes the packet from the
client that generated the exception to be ignored.