connection: keep SYNC messages alive on EINTR

connection: keep SYNC messages alive on EINTR

If a SYNC-SEND is interrupted by a signal, there is no way we can restart
the syscall. If we returned ERESTARTSYS, we'd queue the message again on
restart. This is very irritating, therefore, we never support restarting
syscalls. Instead, we return EINPROGRESS if the message was queued but no
reply was received, yet.

Internally, we turn the 'sync' reply_wait into an 'async' reply. This way,
it will be treated the same way as any other asynchronous reply.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>