Consolidate interface related functions in interface.c

Changes from v1 to v2:
- changed function name prefixes to 'iface' from previous 'Iface'

- Further to make make syntax-check pass:
 - indentation fix in interface.h
 - added entry to POTFILES.in

I am consolidating network interface related functions used in nwfilter
and macvtap code in utils/interface.c. All function names are prefixed
with 'Iface'. The following functions are now available through
interface.h:

int ifaceCtrl(const char *name, bool up);
int ifaceUp(const char *name);
int ifaceDown(const char *name);

int ifaceCheck(bool reportError, const char *ifname,
               const unsigned char *macaddr, int ifindex);

int ifaceGetIndex(bool reportError, const char *ifname, int *ifindex);

I added 'int ifindex' as parameter to ifaceCheck to the original
function and modified the code accordingly.

po/POTFILES.in

@@ -72,6 +72,7 @@ src/util/bridge.c
 src/util/conf.c
 src/util/hooks.c
 src/util/hostusb.c
+src/util/interface.c
 src/util/json.c
 src/util/logging.c
 src/util/macvtap.c