Merge branch 'mn/sideband-no-ansi' into maint

Tools that read diagnostic output in our standard error stream do
not want to see terminal control sequence (e.g. erase-to-eol).
Detect them by checking if the standard error stream is connected
to a tty.

* mn/sideband-no-ansi:
  sideband.c: do not use ANSI control sequence on non-terminal