dns/rfc2136: delete cache files if nsupdate failed

[perryflynn]

See #4055.

Similar to #2752, updating my DynDNS Domain via the rfc2136 plugin does not work. I added some log lines to the plugin code and it looks like the update fails at rc.bootup because the internet connection is not established yet. Later on rc.newwanip the plugin reports the IP was not changed and the nsupdate call is skipped.

My workaround is to delete the cache files when the exit code of the nsupdate command is not zero.

[perryflynn]

@AdSchellevis already mentioned in the issue that here a background process should be used. How can I use a background process here and get the exit code to decide if the cache file should be deleted?

Does mwexec_bg support shell pipes? Then I could do something like this:

$cmd .= " /var/etc/nsupdatecmds{$i}";
$cmd .= " || rm -f ".rfc2136_cache_file($dnsupdate, 4)." ".rfc2136_cache_file($dnsupdate, 6);