lib: move all UNITTEST prototypes to C files

- make extract-unit-protos handle multi-line prototypes - but they need
  to be above the implementation

- Prototypes for static functions we use in unit tests should not be in
  header files. We generate lib/unitprotos.h for this purpose

- Removed some function wrappers written for unit tests and make them
  use UNITTEST function directly.

- Renamed time2str() in the tool to timebuf() since we have the same
  name in lib/ and in unit tests they can both be used non-static in a
  build.

This reverts commit f95fadd116.

Follow-up to #21010

Closes #21014

src/tool_progress.h

@@ -39,7 +39,7 @@ void progress_finalize(struct per_transfer *per);
 
 #ifdef UNITTESTS
 UNITTEST char *max5data(curl_off_t bytes, char *max5, size_t mlen);
-UNITTEST void time2str(char *r, size_t rlen, curl_off_t seconds);
+UNITTEST void timebuf(char *r, size_t rlen, curl_off_t seconds);
 #endif
 
 #endif /* HEADER_CURL_TOOL_PROGRESS_H */