ofctl_v1_2/3: IPv4 dotted decimal subnet mask support

This enables to_match_ip() to accept IPv4 address with dotted decimal
subnet mask or ACL hybrid CIDR.

Given 3 match field values below:

    '192.168.1.0/24'
    '192.168.1.0/255.255.255.0'
    '192.168.1.0/0.0.0.255'

These addresses are logically equivalent.

Signed-off-by: Wei-Li Tang <alextwl@xinguard.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>