dhcp: fix shutdown not work by SIGTERM while bind9 enable threads

dhcp: fix shutdown not work by SIGTERM while bind9 enable threads

In https://source.isc.org/git/bind9.git, since the following
commit applied:
...
commit b99bfa184bc9375421b5df915eea7dfac6a68a99
Author: Evan Hunt <each@isc.org>
Date:   Wed Apr 10 13:49:57 2013 -0700

    [master] unify internal and export libraries

    3550.       [func]          Unified the internal and export versions of the
                        BIND libraries, allowing external clients to use
                        the same libraries as BIND. [RT #33131]
...
(git show b99bfa184bc9375421b5df915eea7dfac6a68a99 -- ./lib/isc/unix/app.c)

In this commit, if bind9 enable threads(ISC_PLATFORM_USETHREADS),
it blocks signal SIGHUP, SIGINT and SIGTERM in isc__app_ctxstart.
Which caused dhclient/dhcpd could not be stopped by SIGTERM.

It caused systemd's reboot hung which send SIGTERM by default.

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>