fftools/ffmpeg: replace MATCH_PER_STREAM_OPT(.., str, ..) with a function

This has multiple advantages:
* The macro has multiple parameters that often have similar or identical
  values, yet very different meanings (one is the name of the
  OptionsContext member where the parsed options are stored, the other
  the name of the variable into which the result is written); this
  change makes each of these explicit.

* The macro returns on failure, which may cause leaks - this was the
  reason for adding MATCH_PER_STREAM_OPT_CLEAN(), also ost_add()
  currently leaks encoder_opts. The new function returns failure to its
  caller, which decides how to deal with it. While that adds a lot of
  error checks/forwards for now, those will be reduced in following
  commits.

* new code is type- and const- correct

Invocations of MATCH_PER_STREAM_OPT() with other types will be converted
in following commits.

fftools/ffmpeg.h

@@ -593,7 +593,7 @@ typedef struct OutputStream {
 
     KeyframeForceCtx kf;
 
-    char *logfile_prefix;
+    const char *logfile_prefix;
     FILE *logfile;
 
     // simple filtergraph feeding this stream, if any
@@ -902,6 +902,8 @@ void update_benchmark(const char *fmt, ...);
 
 const char *opt_match_per_type_str(const SpecifierOptList *sol,
                                    char mediatype);
+int opt_match_per_stream_str(void *logctx, const SpecifierOptList *sol,
+                             AVFormatContext *fc, AVStream *st, const char **out);
 
 int muxer_thread(void *arg);
 int encoder_thread(void *arg);