fd: Open-code para_chdir().

fd: Open-code para_chdir().

Another public trivial wrapper that can go away because it has only
a single caller. POSIX says

	Upon successful completion, 0 shall be returned. Otherwise, −1 shall
	be returned, the current working directory shall remain unchanged,
	and errno shall be set to indicate the error.

So the new check against zero is equivalent to the old code which
checked whether the return value is non-negative.