Update netfront so that it queries and honors published
back-end features.

sys/dev/xen/netfront/netfront.c:
o Add xn_query_features() which reads the XenStore and

	  records the TSO, LRO, and chained ring-request support
	  of the backend.

o Rename xn_configure_lro() to xn_configure_features() and

	  use this routine to manage the setup of TSO, LRO, and
	  checksum offload.

o In create_netdev(), initialize if_capabilities and

	  if_hwassist to the capabilities found on all backends.
	  Delegate configuration of if_capenable and the TSO flag
	  if if_hwassist to xn_configure_features().

Reported by: Hugo Silva (fix inspired by patch provided)
Approved by: re
MFC after: 1 week