Import snmpd_metrics.

This contains snmpd's mib.c (and friends) adjusted for libagentx.
This standalone binary is to be used by snmpd to achieve privilege
separation.

If people need net-snmpd, but

Import snmpd_metrics.

This contains snmpd's mib.c (and friends) adjusted for libagentx.
This standalone binary is to be used by snmpd to achieve privilege
separation.

If people need net-snmpd, but want some of the base snmpd metrics they can
start this binary as a normal daemon and connect to net-snmpd's agentx
socket.

Tested, Feedback, and OK sthen@
Release build test, and OK tb@

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