dns/nsd RC script patch

[Darren Pilgrim]

Jaap Akkerhuis asked that I post here for discussion of a patch I 
emailed to him for the NSD port.  The patch is question may be found here:

http://shibboleet.com/files_nsd.in.patch.txt

The patch adds the ability to run multiple NSD instances using an eval 
hack I originally saw in the isc-dhcpd RC script.

Changes:

- Introduce a new RC variable, ${name}_conf, which defaults to the
current, built-in value (%%PREFIX%%/etc/nsd/nsd.conf) for the base case;
- Add extra RC commands for the notify, patch, rebuild and update nsdc
commands.

The "added complexity" is two parts:

1. Add logic to get the basename used to invoke the script and rename 
nsd_enable and nsd_conf to match.

2. Make the RC script the single point of control for NSD instances. 
Add the rest of the nsdc commands to the RC script's extra_commands. 
Remembering which config file is which instance might be bothersome, so 
instead of doing:

nsdc -c /usr/local/etc/nsdfoo.conf rebuild

You can now just do:

/usr/local/etc/rc.d/nsdfoo rebuild

Adding an instance named "nsdfoo":

1. Create the NSD config (default is /usr/local/etc/nsd/nsdfoo.conf). 
It will need a separate pidfile, database, difffile and xfrdfile.  You 
should also have separate zonesdir locations, but they can be shared in 
some cases.
2. Add 'nsdfoo_enable="YES"' to /etc/rc.conf.
3. cd /usr/local/etc/rc.d && ln -s nsd nsdfoo
4. /usr/local/etc/rc.d/nsdfoo start

If you don't want to keep all your instance configs in the same 
directory (or just not follow the default naming), then replace step 2 with:

2. Add 'nsdfoo_conf="/path/to/conf"' and 'nsdfoo_enable="YES"' to 
/etc/rc.conf.