MINOR: sock: add EPERM case in sock_handle_system_err

MINOR: sock: add EPERM case in sock_handle_system_err

setns() may return EPERM if thread, that tries to move into different
namespace, do not have CAP_SYS_ADMIN capability in its Effective set.
So, extending sock_handle_system_err() with this error allows to send
appropriate log message and set SF_ERR_PRXCOND (SC termination
flag in log) as stream termination error code. This error code can be
simply checked with SF_ERR_MASK at protocol layer.