OPNSense/core 7a15556src/etc/inc/plugins.inc.d dhcpd.inc, src/www services_dhcp.php

minor cleanups for https://github.com/opnsense/core/pull/3484 and handle 0 values.
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+8-5src/www/services_dhcp.php
+2-3src/etc/inc/plugins.inc.d/dhcpd.inc
+10-82 files

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