tree-wide: make parse_ifindex simply return the index

We don't need a seperate output parameter that is of type int.  glibc() says
that the type is "unsigned", but the kernel thinks it's "int".  And the
"alternative names" interface also uses ints. So let's standarize on ints,
since it's clearly not realisitic to have interface numbers in the upper half
of unsigned int range.

src/basic/parse-util.c

@@ -79,11 +79,10 @@ int parse_mode(const char *s, mode_t *ret) {
         return 0;
 }
 
-int parse_ifindex(const char *s, int *ret) {
+int parse_ifindex(const char *s) {
         int ifi, r;
 
         assert(s);
-        assert(ret);
 
         r = safe_atoi(s, &ifi);
         if (r < 0)
@@ -91,26 +90,24 @@ int parse_ifindex(const char *s, int *ret) {
         if (ifi <= 0)
                 return -EINVAL;
 
-        *ret = ifi;
-        return 0;
+        return ifi;
 }
 
-int parse_ifindex_or_ifname(const char *s, int *ret) {
+int parse_ifindex_or_ifname(const char *s) {
         int r;
 
         assert(s);
-        assert(ret);
 
-        r = parse_ifindex(s, ret);
-        if (r >= 0)
+        r = parse_ifindex(s);
+        if (r > 0)
                 return r;
+        assert(r < 0);
 
         r = (int) if_nametoindex(s);
         if (r <= 0)
                 return -errno;
 
-        *ret = r;
-        return 0;
+        return r;
 }
 
 int parse_mtu(int family, const char *s, uint32_t *ret) {