net-snmp: "extend" module config support

[MorphBonehunter]

Maintainer: @stintel
Environment: (MIPS, TP-Link TL-WR841N/ND v10)

Description:
With #4085 and ec6712f the "extend" module was introduced in the package but the UCI doesn't know anything about it.
So i would suggest to extend the UCI with the possibility to configure the extend module.
As the snmpd grumbles about the exec line in the config:

Error: WARNING: This output format is not valid, and is only retained for backward compatibility - Please consider using the 'extend' directive instead

this should maybe replace the exec in general.
So atm i didn't make an PR but i want to share the following suggestion (don't know if this is the right way):

diff --git a/net/net-snmp/files/snmpd.init b/net/net-snmp/files/snmpd.init
index caffecf6..bc0f70d8 100644
--- a/net/net-snmp/files/snmpd.init
+++ b/net/net-snmp/files/snmpd.init
@@ -175,7 +175,7 @@ snmpd_pass_add() {
        echo "$pass $priority $miboid $prog" >> $CONFIGFILE
 }
 
-snmpd_exec_add() {
+snmpd_extend_add() {
        local cfg="$1"
 
        config_get name "$cfg" name
@@ -184,7 +184,7 @@ snmpd_exec_add() {
        [ -n "$prog" ] || return 0
        config_get args "$cfg" args
        config_get miboid "$cfg" miboid
-       echo "exec $miboid $name $prog $args" >> $CONFIGFILE
+       echo "$1 $miboid $name $prog $args" >> $CONFIGFILE
 }
 
 snmpd_disk_add() {
@@ -293,7 +293,8 @@ start_service() {
        config_foreach snmpd_access_HostName_add access_HostName
        config_foreach snmpd_access_HostIP_add access_HostIP
        config_foreach snmpd_pass_add pass
-       config_foreach snmpd_exec_add exec
+       config_foreach snmpd_extend_add extend
+       config_foreach snmpd_extend_add exec
        config_foreach snmpd_disk_add disk
        config_foreach snmpd_engineid_add engineid
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