fixup i/o handler return values

fixup i/o handler return values

Particularly important for lxc_cmd_handler() handles client
input and should not be capable of canceling the main loop,
some syscall return values leaked through overlapping with
LXC_MAINLOOP_ERROR, causing unauthorized clients connecting
to the command socket to shutdown the main loop.

In turn, signal_handler() receiving unexpected
`signalfd_siginfo` struct sizes seems like a reason to bail
(since it's a kernel interface).

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>