posix_fildes(): New helper: run a function that takes a file descriptor

posix_fildes():  New helper: run a function that takes a file descriptor
    and returns None.  This allows any object that supports the fileno()
    method to be passed as a file descriptor, not just an integer.

posix_fchdir():  New exposed function: implements posix.fchdir().  This
    closes SF feature #536796.

posix_fsync(), posix_fdatasync():  Convert to use posix_fildes() instead
    of posix_int().  This also changes them from METH_VARARGS to METH_O
    functions.

setup_confname_table():  Remove unused variable.  Change to take a module
    rather than a dict to save the resulting table into.

setup_confname_tables():  Change to take a module instead of a dict to
    pass to setup_confname_table().