xtables-translate: Fix for interface name corner-cases

xtables-translate: Fix for interface name corner-cases

There are two special situations xlate_ifname() didn't cover for:

* Interface name containing '*': This went unchanged, creating a command
  nft wouldn't accept. Instead translate into '\*' which doesn't change
  semantics.

* Interface name being '+': Can't translate into nft wildcard character
  as nft doesn't accept asterisk-only interface names. Instead decide
  what to do based on 'invert' value: Skip match creation if false,
  match against an invalid interface name if true.

Also add a test to make sure future changes to this behaviour are
noticed.

Signed-off-by: Phil Sutter <phil@nwl.cc>