Make more SNMP trap special variables available to the SNMP trap parsers

Most importantly, this commit adds the `$W` special variable which is the SNMP trap description, if available.

However, for this to work the `/etc/snmp/snmptrapd.conf` file must contain the format execute option as follows:

```
authCommunity log,execute,net public
format execute $a %a\n$A %A\n$b %b\n$B %B\n$N %N\n$q %q\n$P %P\n$t %t\n$T %T\n$w %w\n$W %W\n%V~\%~%v\n
traphandle default alert-snmptrap --debug
```