all: use designated initializers for AVOption.unit

Makes it robust against adding fields before it, which will be useful in
following commits.

Majority of the patch generated by the following Coccinelle script:

@@
typedef AVOption;
identifier arr_name;
initializer list il;
initializer list[8] il1;
expression tail;
@@
AVOption arr_name[] = { il, { il1,
- tail
+ .unit = tail
}, ...  };

with some manual changes, as the script:
* has trouble with options defined inside macros
* sometimes does not handle options under an #else branch
* sometimes swallows whitespace

libavfilter/avfilter.c

@@ -616,7 +616,7 @@ static const AVClass *filter_child_class_iterate(void **iter)
 #define TFLAGS AV_OPT_FLAG_FILTERING_PARAM|AV_OPT_FLAG_RUNTIME_PARAM
 static const AVOption avfilter_options[] = {
     { "thread_type", "Allowed thread types", OFFSET(thread_type), AV_OPT_TYPE_FLAGS,
-        { .i64 = AVFILTER_THREAD_SLICE }, 0, INT_MAX, FLAGS, "thread_type" },
+        { .i64 = AVFILTER_THREAD_SLICE }, 0, INT_MAX, FLAGS, .unit = "thread_type" },
         { "slice", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AVFILTER_THREAD_SLICE }, .flags = FLAGS, .unit = "thread_type" },
     { "enable", "set enable expression", OFFSET(enable_str), AV_OPT_TYPE_STRING, {.str=NULL}, .flags = TFLAGS },
     { "threads", "Allowed number of threads", OFFSET(nb_threads), AV_OPT_TYPE_INT,