feat: set up SNMP with net-snmp

run_device_action("fix_snmp") and get_snmp_config for both drivers, as
decided in NetOrk/netork#800: net-snmp from packages rather than the base
daemons, because it answers UCD-SNMP-MIB, which netOrk's health metrics
read.

- fix_snmp installs net-snmp, writes the configuration netOrk uses on
  Linux (v2c, community "public", agentAddress udp:161) through stdin as
  root, enables and restarts the agent (FreeBSD `service snmpd`, OpenBSD
  `rcctl ... netsnmpd`, as `snmpd` is OpenBSD's own daemon) and asks it
  for sysDescr. It stops at the first step that fails and says why.
- get_snmp_config reports a running net-snmp's community and port.
- FreeBSD's install_package bootstraps pkg on a classic system that never
  had it instead of waiting for an answer that never comes.

Checked live: FreeBSD from a bare system, OpenBSD again over an existing
setup. Memory, swap and load answer on both; FreeBSD's ssCpuIdle about a
minute after start; OpenBSD's CPU figures from net-snmp are wrong (#800).

CHANGELOG.md

@@ -8,6 +8,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- SNMP: `run_device_action("fix_snmp")` installs net-snmp, writes the same
+  configuration netOrk uses on Linux, starts the agent and asks it for
+  sysDescr; `get_snmp_config` reports a running agent's community and port.
+
+### Changed
+- FreeBSD's `install_package` bootstraps pkg on a classic system that never
+  had it (`ASSUME_ALWAYS_YES`) instead of waiting for an answer.
 - Packages: `get_packages`, `install_package`, `uninstall_package`.
 - Updates: `get_available_updates`, with VuXML's verdict as `security` on
   FreeBSD and base-system patches as one `base-system` entry (OpenBSD