Replace mktemp() by a new mkifname()

Using mktemp() leads to build time warnings and isn't actually
appropriate for what we want to do as it's checking for the existence of
a file and not a network interface.

Replace those calls by an equivalent mkifname() function which uses the
same template as mktemp but instead checks for existing network
interfaces.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>