fd.c: Avoid gcc warning regarding fchdir().

fd.c: Avoid gcc warning regarding fchdir().

gcc-4.8.4 on Ubuntu-14.04 complains about our use of fchdir():

	fd.c:181:9: warning: ignoring return value of ‘fchdir’, declared with attribute warn_unused_result [-Wunused-result]
	   fchdir(*cwd);

Silence the warning by introducing a dummy variable. The patch also
fixes the indentation of the comment which explains why we ignore
the return value here.