fabric: Rename FI_ADDR_UNSPEC -> FI_FORMAT_UNSPEC

It is the address format that is unspecified, not the address.
This change will allow us to use FI_ADDR_UNSPEC for a
different purpose in a subsequent change, where it will mean
that the address was not specified.

As part of this update, the sockets provider is updated to
return ADDR_NOTAVAIL in place of ADDR_UNSPEC where required.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
fabric: Define FI_ADDR_UNSPEC to mean any source

When posting a receive buffer, allow the user to indicate
that it can match with any source address by using
FI_ADDR_UNSPEC as the src_addr.

Also define an EP capability to indicate whether source
directed receives is required and/or supported by the
provider.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>