xtables-translate: Fix for iface++

In legacy iptables, only the last plus sign remains special, any
previous ones are taken literally. Therefore xtables-translate must not
replace all of them with asterisk but just the last one.

Fixes: e179e87a1179e ("xtables-translate: Fix for interface name corner-cases")
Signed-off-by: Phil Sutter <phil@nwl.cc>