Merge branch 'jc/neuter-sideband-fixup' into next

Merge branch 'jc/neuter-sideband-fixup' into next

Try to resurrect and reboot a stalled "avoid sending risky escape
sequences taken from sideband to the terminal" topic by Dscho.  The
plan is to keep it in 'next' long enough to see if anybody screams
with the "everything dropped except for ANSI color escape sequence"
default.

* jc/neuter-sideband-fixup:
  sideband: drop 'default' configuration
  sideband: offer to configure sanitizing on a per-URL basis
  sideband: add options to allow more control sequences to be passed through
  sideband: do allow ANSI color sequences by default
  sideband: introduce an "escape hatch" to allow control characters
  sideband: mask control characters