unittests: cleanups

- make sure all UNITTEST prototypes mark in which unit test they are used,
  with "@unittest" markup

- make sure all UNITTEST functions do not use Curl_ prefix, as that is a
  prefix we use for global private functions and these functions are static
  and therefore not global and the prefix is wrong

- drop UNITTEST for functions not used in unit tests

- make the extract-unit-protos script highlight the above issues if found

- extract-unit-protos now also outputs the unit test number for all the
  generated protos in lib/unitprotos.h to aid readers. It also adds the source
  file and line number where the proto originates from.

- extract-unit-protos now exits with a non-zero value if any of the above
  warnings are triggered

- cf-dns: Curl_cf_dns_result => static cf_dns_result
- hostip: Curl_ipv6works => static ipv6works
- url: remove Curl_setup_conn() - not used anymore
- connect: Curl_timeleft_now_ms => UNITTEST timeleft_now_ms

Closes #21330

lib/multi.c

@@ -304,10 +304,8 @@ struct Curl_multi *Curl_multi_handle(uint32_t xfer_table_size,
     goto error;
 #endif
 
-#ifdef USE_IPV6
   if(Curl_probeipv6(multi))
     goto error;
-#endif
 
 #ifdef USE_RESOLV_THREADED
   if(xfer_table_size < CURL_XFER_TABLE_SIZE) { /* easy multi */