/* Call this with EITHER (1) "file, NULL, 0" to chdir() to the file's
* F_PATHNAME(), or (2) "NULL, dir, dirlen" to chdir() to the supplied dir,
* with dir == NULL taken to be the starting directory, and dirlen < 0
/* Call this with EITHER (1) "file, NULL, 0" to chdir() to the file's
* F_PATHNAME(), or (2) "NULL, dir, dirlen" to chdir() to the supplied dir,
* with dir == NULL taken to be the starting directory, and dirlen < 0