ntpd: make "unsync" code actually work.

While at it: don't run "periodic" if interrupted by a signal.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
ntpd: set G.last_script_run even if script isn't configured

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge git://busybox.net/busybox