xtables: Introduce and use common function to parse val[/mask] arguments

xtables: Introduce and use common function to parse val[/mask] arguments

There are a couple of places in both core and extensions where arguments
in the form of val[/mask] is parsed (see XTTYPE_MARKMASK32).

In some cases symbolic name might be used which is mapped in code to
numeric value.

Introduce common function to handle both cases where value given is
either val[/mask] or symbolic name.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>