reload /etc/resolv.conf when it has changed

if /etc/resolv.conf is updated, then make sure applications
already running get the updated information.

https://bugs.gentoo.org/177416
https://sourceware.org/bugzilla/show_bug.cgi?id=984
https://sourceware.org/ml/libc-alpha/2004-09/msg00130.html
https://sourceware.org/ml/libc-alpha/2016-12/msg00023.html
https://build.opensuse.org/package/view_file/openSUSE:Factory/glibc/glibc-resolv-reload.diff?expand=1