This is the first of a series of patches to sync with Gnulib commit
615b43e1f9. This patch adopts most of the changes of Gnulib, except it
retains GETCWD_RETURN_TYPE and does not always use a 64-bit internal
API. These remaining discrepancies will be addressed in later patches
in this series.
Checked on x86_64-linux-gnu and i686-linux-gnu.