AVOptions: in av_opt_find() don't return named constants unless unit is specified.

That is, unless the caller explicitly asks for them.

Prevents conflict between e.g. the 'loop' option in img2 demuxer and
'loop' flag in AVCodecContext.

libavutil/opt.c

@@ -573,8 +573,9 @@ const AVOption *av_opt_find(void *obj, const char *name, const char *unit,
         return o;
 
     while (o = av_next_option(obj, o)) {
-        if (!strcmp(o->name, name) && (!unit || (o->unit && !strcmp(o->unit, unit))) &&
-            (o->flags & opt_flags) == opt_flags)
+        if (!strcmp(o->name, name) && (o->flags & opt_flags) == opt_flags &&
+            ((!unit && o->type != FF_OPT_TYPE_CONST) ||
+             (unit  && o->unit && !strcmp(o->unit, unit))))
             return o;
     }
     return NULL;