Hello everyone,

This simple patch adds `$TTL` directive to dns-mode-font-lock-keywords
from the lisp/text-modes/dns-mode.el.

The global `$TTL` directive (time-to-live) specifies the duration in seconds
that the record may be cached by any resolver. In general it should looks:

$TTL amount-of-ttl (in seconds)

Additionally d/D/h/H/w/W/m/M/s/S maybe added to amount-of-ttl where d/D -
days, h/H - hours and etc..

Thank you.