Run as nonroot user on Linux (with CAP_NET_ADMIN and CAP_NET_RAW added).

- ZT will only drop root privileges if zerotier-one user exists. It is created by
Debian postinst script - in other cases the user has to be created by administrator.
- Linux >=4.3 with ambient capabilities is required, otherwise ZT will silently
- "-U" option now also disables privileges dropping

debian/postinst

@@ -0,0 +1,9 @@
+#!/bin/sh -e
+
+case "$1" in
+  configure)
+      adduser --system --group --home /var/lib/zerotier-one --no-create-home zerotier-one
+      ;;
+esac
+
+#DEBHELPER#