Add the missing F_SEAL_SEAL, F_SEAL_SHRINK, F_SEAL_GROW, F_SEAL_WRITE,
F_SEAL_FUTURE_WRITE, and F_SEAL_EXEC constants used with
F.ADD_SEALS/F.GET_SEALS for memfd file sealing. These are defined in the
Linux kernel at include/uapi/linux/fcntl.h.

The FreeBSD equivalents already exist in std.c (freebsd.F),
but the Linux side was missing them.