Use native path separators to pg_ctl in initdb

On Windows, CMD.EXE allegedly does not run a command that uses forward slashes,
so let's convert the path to use backslashes instead.

Backpatch to 10.

Author: Nitin Jadhav <nitinjadhavpostgres@gmail.com>
Reviewed-by: Juan José Santamaría Flecha <juanjo.santamaria@gmail.com>
Discussion: https://postgr.es/m/CAMm1aWaNDuaPYFYMAqDeJrZmPtNvLcJRS++CcZWY8LT6KcoBZw@mail.gmail.com

src/bin/initdb/initdb.c

@@ -3239,6 +3239,9 @@ main(int argc, char *argv[])
 		/* ... and tag on pg_ctl instead */
 		join_path_components(pg_ctl_path, pg_ctl_path, "pg_ctl");
 
+		/* Convert the path to use native separators */
+		make_native_path(pg_ctl_path);
+
 		/* path to pg_ctl, properly quoted */
 		appendShellString(start_db_cmd, pg_ctl_path);