Avoid getcwd(0, PATH_MAX) for GNU libc

Avoid getcwd(0, PATH_MAX) for GNU libc

Recent versions of GNU libc and GCC produce a warning on getcwd(0, PATH_MAX):

 test_main.c: In function ‘get_refdir’:
 test_main.c:3684:8: error: argument 1 is null but the corresponding size argument 2 value is 4096 [-Werror=nonnull]
   3684 |  pwd = getcwd(NULL, PATH_MAX);/* Solaris getcwd needs the size. */

This is because getcwd() is marked with the 'write_only (1, 2)' attribute.

Using the alternate getcwd(NULL, 0) path which is supported by GNU libc avoids this.