packet.ipv4: Fix up IP addrs earlier

In some unusual cases, IPv4 packets' IP addresses were left as raw integers
instead of IPAddrs.  Fixed.

Thanks to eternia478 and nbjsw for the bug report.
util: Add ability to ignore fields to fields_of()
nx: Fix documentation bug

Kept refering to of_tcp_src but meant of_ip_src.  Fixed.
nx: Accept even more magic names in nx_match