parse-util: introduce parse_ifindex() and make use of it everywhere

We have enough places where we parse an ifindex, hence introduce a
proper parsing function for it, that verifies all parameters.

src/basic/parse-util.h

@@ -31,6 +31,7 @@
 int parse_boolean(const char *v) _pure_;
 int parse_pid(const char *s, pid_t* ret_pid);
 int parse_mode(const char *s, mode_t *ret);
+int parse_ifindex(const char *s, int *ret);
 
 int parse_size(const char *t, uint64_t base, uint64_t *size);
 int parse_range(const char *t, unsigned *lower, unsigned *upper);