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

fftools/ffmpeg_mux_init.c

@@ -2215,7 +2215,7 @@ static int of_parse_group_token(Muxer *mux, const char *token, char *ptr)
     const AVDictionaryEntry *e;
     const AVOption opts[] = {
         { "type", "Set group type", offsetof(AVStreamGroup, type), AV_OPT_TYPE_INT,
-                { .i64 = 0 }, 0, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "type" },
+                { .i64 = 0 }, 0, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, .unit = "type" },
             { "iamf_audio_element",    NULL, 0, AV_OPT_TYPE_CONST,
                 { .i64 = AV_STREAM_GROUP_PARAMS_IAMF_AUDIO_ELEMENT },    .unit = "type" },
             { "iamf_mix_presentation", NULL, 0, AV_OPT_TYPE_CONST,