Merge branch 'ps/writev' into seen

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