Move dnsroots.global to etc/servers/.

Moved global root server list - dnsroots.global - to etc/servers/
directory. Updated the Makefile.am there to create - roots - list
using dnsroots.global and bundle `dnsroots.global' in the source
tarball. `roots' file is removed after use.

This would help to maintain a single consistent list of the top
level root servers.