seccomp: keep retrying to reconnect to proxy

seccomp: keep retrying to reconnect to proxy

If a syscall happens after we already failed to communicate
with the proxy, proxy_fd was -1.
Before the previous commit we'd then be stuck in the state
where there was no proxy registered. With the previous
commit we'd send a default reply and only then try to
reconnect.
Improve this even further by trying to reconnect right at
the start.

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