NFD

I propose to add -Wextra to the default compiler flags for ndn-cxx, nfd, and ndn-tools.

The only problem is that it enables the completely useless warning about unused parameters, so we need to disable that at the same time.

Moreover, with gcc-4.x we also have to pass -Wno-missing-field-initializers to suppress a bogus warning that was finally fixed in gcc-5. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36750 and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805