agent: Post merge changes that enable building of vnswd.

Post review fixes.
Merge pull request #11 from Semihalf/dev-upstream-merge

Dev upstream merge
agent: Enabled -Werror and removed warnings that kept failing the build.
Set pkt0 to promisc mode
agent: libxml2 compilation with html has been reenabled.
Merge pull request #15 from Semihalf/fix-libxml-html-enable

agent: libxml2 compilation with html has been reenabled.
vnswad: Fix Linux hosted build of the Agent

A bunch of cleanups and fixes necessary for Linux version of the Agent to
build.
Merge pull request #14 from Semihalf/fix-werror-enabled

agent: Enabled -Werror and removed warnings that kept failing the build.
Merge pull request #16 from Semihalf/dev-cleanup

Dev cleanup
scons: Fix libicu version mismatch between linking-time and run-time

We have to use self-compiled ICU and not the one from the system to ensure
the same C++ ABI (libstdc++ from GNU) to be used by all the libraries
the Agent is linked against.
Merge pull request #17 from Semihalf/dev-icu

scons: Fix libicu version mismatch between linking-time and run-time
agent: Interface/Address/Route change in system observers

This commit includes parsing of PF_ROUTE communication for the
purpose of reacting to in-system changes in Interface, address
and route configuration.
agent: Interface/Address/Route change in system observers

This commit includes parsing of PF_ROUTE communication for the
purpose of reacting to in-system changes in Interface, address
and route configuration.

Post review fixes.