run: Add per-process stdout handlers

Simplify the telnet header stripping by registering a handler for each
process that needs it, instead of a global handler.

This will be necessary in order to set up the terminal processes
one by one: if the terminal starts logging before the handler is
switched to the telnet handler, then we may miss the header and never
output anything.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>