ipaddress: strengthen check on 'label' input

ipaddress: strengthen check on 'label' input

As mentioned in the ip-address man page, an address label must
be equal to the device name or prefixed by the device name
followed by a colon. Currently the only check on this input is
to see if the device name appears at the beginning of the label
string.

This commit adds an additional check to ensure label == dev or
continues with a colon.

Signed-off-by: Patrick Talbert <ptalbert@redhat.com>
Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>