seccomp: use SOCK_SEQPACKET for the notify proxy

seccomp: use SOCK_SEQPACKET for the notify proxy

The seccomp notify API has a few variables: The struct sizes
are queried at runtime, and we now also have a user
configured cookie.
This means that with a SOCK_STREAM connection the proxy
needs to carefully read() the right amount of data based on
the contents of our proxy message struct to avoid ending up
in the middle of a packet.
While for now this may not be too tragic, since we currently
only ever send a single packet and then wait for the
response, we may at some point want to be able to handle
multiple processes simultaneously, hence it makes sense to
switch to a packet based connection.

So switch to using SOCK_SEQPACKET which is packet based,
(and also guarantees ordering). The `MSG_PEEK` flag can be
used with `recvmsg()` to figure out a packet's size on the
other end, and usually the size *should* not change after
that for an existing connection from a running container.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>