[$] Zero-copy networking

In many performance-oriented settings, the number of times that data iscopied puts an upper limit on how fast things can go. As a result,zero-copy algorithms have long been of interest, even though the benefitsachieved in practice tend to be disappointing. Networking is often performance-sensitive and is definitely dominated bythe copying of data, so an interest in zero-copy algorithms in networkingcomes naturally. A set of patches under review makes that capabilityavailable, in some settings at least.