nfs: Add support for the NFSv4.2 mode_umask attribute

RFC8275 defines a new attribute as an extension to NFSv4.2
called MODE_UMASK. This patch adds support for this attribute
to the NFSv4.2 client and server.

Since FreeBSD applies the umask above the VFS/VOP layer,
this attribute does not actually have any effect on the
handling of ACL inheritance, which is what it is designed for.
However, future changes to NFSv4.2 require support of it,
so this patch does that, resulting in behaviour identcal to
the mode attribute already supported.

MFC after: 2 months