BUILD: build with -Wextra and sort out certain warnings

BUILD: build with -Wextra and sort out certain warnings

We're not far from being able to build with -Wextra -Werror. The
following warnings had to be disabled to enable a clean build at
-Wextra on x86_64 using gcc 4.7, 5.5, 6.4 and 7.3 :

   sign-compare, unused-parameter, old-style-declaration,
   ignored-qualifiers, clobbered, missing-field-initializers,
   implicit-fallthrough

The following extra warnings could be added without side effects :

   type-limits, shift-negative-value, shift-overflow=2 duplicated-cond,
   null-dereference

As a result, -Wextra was enabled by default, hoping it will help catch
issues over the long term. If new undesired warnings pop up, it's easy
to disable them using the nowarn call.