bpo-38353: Add subfunctions to getpath.c (pythonGH-16572)

Following symbolic links is now limited to 40 attempts, just to
prevent loops.

Add subfunctions:

* Add resolve_symlinks()
* Add calculate_argv0_path_framework()
* Add calculate_which()
* Add calculate_program_macos()

Fix also _Py_wreadlink(): readlink() result type is Py_ssize_t, not
int.

Author: vstinner