Merge branch 'jc/no-writev-does-not-work'

Merge branch 'jc/no-writev-does-not-work'

We used writev() in limited code paths and supplied emulation for
platforms without working writev(), but the emulation was too
faithful to the spec to make the result useless to send even 64kB;
revert the topic and plan to restart the effort later.

* jc/no-writev-does-not-work:
  Revert "compat/posix: introduce writev(3p) wrapper"
  Revert "wrapper: introduce writev(3p) wrappers"
  Revert "sideband: use writev(3p) to send pktlines"
  Revert "cmake: use writev(3p) wrapper as needed"