access.2, chmod.2, chown.2, execveat.2, futimesat.2, link.2, mkdir.2, mknod.2, mount_...

access.2, chmod.2, chown.2, execveat.2, futimesat.2, link.2, mkdir.2, mknod.2, mount_setattr.2, open.2, open_by_handle_at.2, readlink.2, rename.2, stat.2, statx.2, symlink.2, unlink.2, utimensat.2, mkfifo.3, scandir.3: Fix EBADF error description

Make the description of the EBADF error for invalid 'dirfd' more
uniform. In particular, note that the error only occurs when the
pathname is relative, and that it occurs when the 'dirfd' is
neither valid *nor* has the value AT_FDCWD.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>