Port recent rename changes to RHEL 7 + NFS

Port recent rename changes to RHEL 7 + NFS

Problem reported by Ted Zlatanov in:
http://lists.gnu.org/archive/html/emacs-devel/2017-08/msg00082.html
* src/fileio.c (Frename_file): On RHEL 7 + NFS, renameat2 can fail
with errno == EINVAL when it is not supported.  So treat that case
like errno == ENOSYS.  Also, when ok_if_already_exists is neither
nil nor an integer, just call plain rename; this avoids an extra
syscall to renameat2 when the latter fails with errno == EINVAL or
ENOSYS or ENOENT.