dhcpv6-ia: make tmp lease file hidden

dhcpv6-ia: make tmp lease file hidden

Use a hidden . prefixed temporary lease file instead of appending
'.tmp'.  Dnsmasq is capable of scanning files/directories using inotify
to receive file change notifications and updating its view of hostname
ip address mapping without being SIGHUPped.  Until dnsmasq v2.88 this
mechanism allows additions to hostnames, no deletions.  dnsmasq v2.88
when released will understand how to remove mappings.

Unfortunately without this change dnsmasq sees odhcpd's temporary lease
file via inotify and it also sees the change when odhcpd atomically
renames the file from '.tmp' to the correct name.

dnsmasq excludes hidden '.' files from it's inotify scans, thus changing
odhcpd to use a hidden temporary lease file reduces load and makes
sense.

Also, while here, only rename the temporary file if it actually contains
different content.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>