harden Linux seccomp sandbox

Linux mmap(2) and madvise(2) syscalls support quite a number of funky
flags that we don't expect that sshd/libc will ever need. We can
exclude this kernel attack surface by filtering the mmap(2) flags
and the madvise(2) advice arguments.

Similarly, the sandboxed process in sshd is a single-threaded program
that does not use shared memory for synchronisation or communication.
Therefore, there should be no reason for the advanced priority
inheritance futex(2) operations to be necessary. These can also be
excluded.

Motivated by Jann Horn pointing out that there have been kernel bugs
in nearby Linux kernel code, e.g. CVE-2020-29368, CVE-2020-29374 and
CVE-2022-42703.

Feedback Jann Horn, ok dtucker@
