Optimizes the creation of a returned payload message by duplicating
the original message and then updating the small number of fields
that need to be adjusted, rather than building the new message header
from scratch. In addition, certain operations that are not always
required are relocated so that they are only done if needed.

These optimizations also have the effect of addressing other issues
that were present previously:

1) Fixes a bug that caused the socket send routines to return the
size of the returned message, rather than the size of the sent
message, when a returnable payload message was sent to a non-existent
destination port.

2) The message header of the returned message now matches that of
the original message more closely. The header is now always the same
size as the original header, and some message header fields that
weren't being initialized in the returned message header are now
populated correctly -- namely the "d" and "s" bits, and the upper
bound of a multicast name instance (where present).
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>