Fix shown IP address belonging to wrong interface

Fix shown IP address belonging to wrong interface

The following commit:

  6a75ea9 Show IP address when address has a label

introduced a way to show the IP address of an interface when a label is
associated to the IP.

When a label is associated to an IP, the structure returned by
getifaddrs() has the label concatenated to the interface name in the
.ifa_name field as in the following example:

    struct ifaddrs ifaddr = {
       .ifa_name = "eth0:mylabel",
    };

As a consequence, using a strict comparison between the interface name
and the .ifa_name field yields a falsy result. However, checking if the
.ifa_name starts with the interface name (e.g. eth0) does not work
either because other network interfaces can have a name which starts
with eth0.

This commit solves the issue by stripping out the optional label from
the .ifa_name field before making a strict comparison with the interface
name.

Fix #283

Signed-off-by: Olivier Gayot <olivier.gayot@sigexec.com>