assertions and fixes recommended by Frama-C

all tests pass on 64bit
config.mak: disable -Werror, -Wno-zero-length-array -Wno-gnu
PN_F: do not return volatile long

volatile makes no sense as function return type, and gcc/clang warn about it.
warning: function return types not compatible due to volatile.
esp with -Werror

move PN_F up as it is used earlier