dns-resolve: Improve on handling of system DNS nameserver

dns-resolve: Improve on handling of system DNS nameserver

This patch enables OVS on windows to read system nameserver configuration.
In addition, a new environment variable OVS_RESOLV_CONF is introduced.
If set, it can be used as DNS server configuration file. This variable
is supposed to be used for sandboxing other things. It is documented
accordingly.

Suggested-by: Ben Pfaff <blp@ovn.org>
Suggested-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>