gre: add different per-protocol prefixes to GRE-TAP IPv4/6 tunnel interfaces.

gre: add different per-protocol prefixes to GRE-TAP IPv4/6 tunnel interfaces.

This commit modifies the /lib/netifd/proto/gre.sh script so that, when
GRE-TAP tunnels are created, either IPv4 or IPv6, the prefix before the chosen
interface name contains the "tap" substring, to differentiate them from non-TAP
GRE tunnels.

Right now, both GRE and GRE-TAP tunnel (either IPv4 or IPv6) interfaces defined
in /etc/config/network are named equally ("gre-"+$ifname or "grev6"+$ifname)
upon creation. For instance, the following tunnels:

        config interface 'tuna'
                option peeraddr '172.30.22.1'
                option proto 'gre'

        config interface 'tunb'
                option peeraddr '192.168.233.4'
                option proto 'gretap'

        config interface 'tunc'
                option peer6addr 'fdc5:7c9e:e93d:45af::1'
                option proto 'grev6'

        config interface 'tund'
                option peer6addr 'fdc0:6071:1348:31ff::2'
                option proto 'grev6tap'

are named, respectively, "gre-tuna", "gre-tunb", "grev6-tunc" and "grev6-tund".

The current change makes that each GRE tunnel interface of the four different
types available (gre, gretap, grev6 and grev6tap) gets a different prefix.
Therefore, the abovementioned tunnels will be named, respectively:
"gre4-tuna", "gre4t-tunb", "gre6-tunc" and "gre6t-tund".

This is coherent with other types of virtual interfaces (i.e. PPP, PPPoE, PPPoA)
where the whole protocol name is used. For instance, a PPPoA interface named
"p1" and a PPPoE interface named "p2" will respectively appear as "pppoa-p1"
and "pppoe-p2", not as "ppp-p1" and "ppp-p2").

Since Linux interfaces names are limited to 15 characters, these prefixes leave,
for the worst case (TAP tunnels), 9 characters for the actual name.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>