luci-mod-network: Fix 'instances' fields for dnsmasq dhcp config entries

Fixed error wherein luci erroneously saved the iterator integer of the
current dnsmasq config object to a host (and boot/PXE) config entry
'instance' field, instead of correctly referring to its name.

Now we use the correct ".name" field of the dnsmasq config entry.
Anonymous entries have e.g. "cfg01411c". The ".name" field corresponds
to 'myName' in /etc/config/dhcp entries of:

config dnsmasq 'myName'
  ...

In this way, host and other entry types are bound correctly to specific
dnsmasq instances. For anonymous entries, display "dnsmasq[x]" as name.

Signed-off-by: Paul Donald <newtwen@gmail.com>