Implemented get_default_gateway_mac_addr for Mac OS X (previously,

Implemented get_default_gateway_mac_addr for Mac OS X (previously,
was only defined for Windows and Linux).  This enables OS X to report
the MAC address of the default gateway to the server for ID purposes
when client-side --push-peer-info option is specified.

Also, minor fix to OS X get_default_gateway function:

* include net/route.h directly rather than selectively paste stuff
  from it into route.c

git-svn-id: http://svn.openvpn.net/projects/openvpn/branches/BETA21/openvpn@6925 e7ae566f-a301-0410-adde-c780ea21d3b5