BUG/MEDIUM: log: emit '-' for empty fields again

Commit 2b0108ad accidently got rid of the ability to emit a "-" for
empty log fields. This can happen for captured request and response
cookies, as well as for fetches. Since we don't want to have this done
for headers however, we set the default log method when parsing the
format. It is still possible to force the desired mode using +M/-M.
diff --git a/include/proto/log.h b/include/proto/log.h
index 72b9f2b..08a9a1b 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -70,7 +70,7 @@
  * Variable name are preceded by % and composed by characters [a-zA-Z0-9]* : %varname
  * You can set arguments using { } : %{many arguments}varname
  */
-void parse_logformat_string(const char *str, struct proxy *curproxy, struct list *list_format, int capabilities);
+void parse_logformat_string(const char *str, struct proxy *curproxy, struct list *list_format, int options);
 /*
  * Displays the message on stderr with the date and pid. Overrides the quiet
  * mode during startup.