net-snmp: fix service not restarting on config change

net-snmp: fix service not restarting on config change

snmpd generates its runtime configuration in /var/run/snmpd.conf from
UCI during start_service(). However, since the procd instance command
line never changes, procd does not detect that a restart is needed when
the UCI config is modified.

Add 'procd_set_param file /etc/config/snmpd' so procd tracks the config
file and restarts snmpd when it changes. Without this, 'reload_service'
(triggered by procd_add_reload_trigger) re-creates an identical instance
definition and procd skips the restart, leaving stale configuration
active.

This also fixes a usability issue with SNMPv3: when changing a user's
authentication or privacy algorithm, net-snmp must restart to re-derive
localized keys via createUser. Without the restart, the daemon keeps
using cached key material and authentication fails.

Tested with net-snmp 5.9.4 on OpenWrt (aarch64 and ppc64).

Signed-off-by: Michael Pfeifroth <redacted>