iptables: accept lock file name at runtime

iptables: accept lock file name at runtime

allow users to override at runtime the lock file to use through the
XTABLES_LOCKFILE environment variable.

It allows to use iptables when the user has granted enough
capabilities (e.g. a user+network namespace) to configure the network
but that lacks access to the XT_LOCK_NAME (by default placed under
/run).

$ XTABLES_LOCKFILE=/tmp/xtables unshare -rn iptables ...

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>