Merge branch 'ps/writev'

Merge branch 'ps/writev'

A compatibility wrapper for writev(3p) has been reintroduced,
including fixes for CMake build and 'MAX_IO_SIZE' limits on NonStop.
Calls to write(3p) in send_sideband() and cat_blob() have been
refactored to use writev(3p) wrappers to reduce syscall overhead.

* ps/writev:
  fast-import: use writev(3p) to send cat-blob responses
  sideband: use writev(3p) to send pktlines
  wrapper: properly handle MAX_IO_SIZE in writev(3p)
  wrapper: introduce writev(3p) wrappers
  compat/posix: introduce writev(3p) wrapper