3825: write log output directly to a pipe.

Added functions:
* start_output_log($logfilename)
* write_output_log($txt)
* finish_output_log()
* output_log_is_active()

Rewrote all code that referenced $local_logfile in terms of these
functions.
3825: code review

* avoid overloading "output" (usually used for the output from a task or job than for diagnostic output from crunch)
** renamed:
*** start_output_log -> log_writer_start
*** write_output_log -> log_writer_send
*** finish_output_log -> log_writer_finish
*** output_log_is_active -> log_writer_is_active
* fixed missing semicolon
Merge branch '3825-crunch-pipe-to-arv-put-final'

Closes #3825.