dnsmasq: rework jail mounts

dnsmasq: rework jail mounts

 * split into multiple lines to improve readability
 * use EXTRA_MOUNT for addnhosts instead of blindly adding /tmp/hosts
 * remove no longer needed mount for /sbin/hotplug-call
 * add dhcp-script.sh dependencies (jshn, ubus)

Fixes: 3a94c2ca5c ("dnsmasq: add /tmp/hosts/ to jail_mount")
Fixes: aed95c4cb8 ("dnsmasq: switch to ubus-based hotplug call")
Reported-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 59c63224e11d6c4eca27131a73bf16218e47a271)