Performance improvements, more test modes

- mmapped transfer functions now use an additional buffer to place the data
  in a cacheable area before sending them out. increases rate to >50MB/s for
  transfer over sockets and to ~120MB/s for writing to RAMdisk
- pipe-based transfer test functions. if our buffers turn out to be
  irredeemably un-vmsplice()able, it's for the bin
- added file-name parameter (-f,--fname) for file target