Merge from gnulib.

This incorporates:
2014-10-18 readlinkat: port to AIX 7.1
2014-10-07 fcntl: fix error reporting by dupfd
* lib/fcntl.c, lib/readlinkat.c, lib/unistd.in.h, m4/readlinkat.m4:
* m4/unistd_h.m4: Update from gnulib.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.