Plug a fd leak, add a bunch of warnings and fix minor things

[haesbaert]

• Get_auxiliary_vector_base() leaks an fd as it never calls fclose()
• Fix a bunch of signed vs unsigned
• Use ssize_t and size_t where deemed
• Avoid void * arithmetic, while gcc and clang capitulated and implicit cast to char *, it's still undef behavior by the standard.
• Declare some remaining prototypes as void
• Add the following to CFLAGS only on our code (non-GPL/*)
• Consistently use NLMSG_TAIL, I think there is one remaining bug where one caller doesn't align the length up, but I'd address that in a separate PR, I don't this diff to be too invasive.

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wextra")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wchar-subscripts")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wcomment")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wformat")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wformat-security")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wimplicit")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Winline")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-declarations")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-prototypes")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wparentheses")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wpointer-arith")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wreturn-type")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wshadow")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wsign-compare")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wstrict-prototypes")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wswitch")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wtrigraphs")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wuninitialized")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wunused")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-parameter")

[Tacklebox]

LGTM