dnsmasq: write atomic host file

dnsmasq: write atomic host file

Different invocations of the dnsmasq init script (e.g. at startup by procd)
will rewrite the dhcp host file which might result into dnsmasq reading an
empty dhcp host file as it is being rewritten by the dnsmasq init script.
Let the dnsmasq init script first write to a temp dhcp host file so it does
not overwrite the contents of the existing dhcp host file.

Reported-by: Hartmut Birr <e9hack@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>