unshare.2: Same EINVAL errors as for clone(2) can also occur with unshare(2)

unshare.2: Same EINVAL errors as for clone(2) can also occur with unshare(2)

The EINVAL errors that can occur for clone(2) when it is called
with various CLONE_NEW* flags and the kernel was not configured
with support for the corresponding namespace can also occur for
unshare(2).  (As far as I can see, these errors don't occur for
either clone(2) or unshare(2) when it comes to CLONE_NEWNS and
CLONE_NEWCGROUP.)

Reported-by: Shawn Landden <shawn@git.icu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>