In snmpd_metrics process keep file descriptors 0 1 2 open.

In snmpd_metrics process keep file descriptors 0 1 2 open.

stdin, stdout, stderr are reserverd.  They should point to a terminal,
a redirect file or pipe, or /dev/null.  Closing them and allocating
arbitrary files in those places is not good.  Call closefrom with
4 to keep /dev/null open in snmpd_metrics.

OK martijn@