readlinkat: avoid OS X 10.10 trailing slash bug * doc/posix-functions/readlink.texi: Mention that OS X 10.10 has this bug. * doc/posix-functions/readlinkat.texi: Likewise. Also mention that OS X 10.10 has this function. * lib/readlinkat.c (rpl_readlinkat): Handle the trailing slash bug, as done for readlink(). * m4/readlinkat.m4 (gl_FUNC_READLINKAT): Check for the readlink() trailing slash bug, and assume readlinkat() has the same issue. Also fix a typo where $gl_cv_decl_readlink_works was tested, rather than the correct $gl_cv_decl_readlinkat_works.