Add general-purpose IP/port parsing function.

Add general-purpose IP/port parsing function.

OVS has functions for parsing IPv4 addresses, parsing IPv4 addresses
with a port, and parsing IPv6 addresses. What is lacking though is a
function that can take an IPv4 or IPv6 address, with or without a port.

This commit adds ipv46_parse(), which breaks the given input string into
its component parts and stores them in a sockaddr_storage structure. The
function accepts flags that determine how it should behave if a port is
present in the input string.

Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>