libxtables: prefix/order - fw_xalloc

It is good practice to prefix names in a library some way so that
it does not clash with external programs' variable names right
on the first try.

This change: rename fw_[cm]alloc to xtables_[cm]alloc and
move the definition from internal.h to xtables.h to avoid
potential compiler warnings.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
6 files changed