status: Build upon multiplexed build output.

status: Build upon multiplexed build output.

This allows for more accurate status tracking and parsing of extended
build traces.

* guix/status.scm (multiplexed-output-supported?): New procedure.
(print-build-event): Don't print \r when PRINT-LOG? is true.
Adjust 'build-log' handling for when 'multiplexed-output-supported?'
returns true.
(bytevector-index, split-lines): New procedures.
(build-event-output-port)[%build-output-pid, %build-output]
[%build-output-left]: New variables.
[process-line]: Handle "@ build-output" traces.
[process-build-output]: New procedure.
[write!]: Add case for when %BUILD-OUTPUT-PID is true.  Use
'bytevector-index' rather than 'string-index'.
(compute-status): Add #:derivation-path->output-path.  Use it.
* tests/status.scm ("compute-status, multiplexed build output"):
New test.
("build-output-port, UTF-8")
("current-build-output-port, UTF-8 + garbage"): Adjust to new
'build-log' output.
* guix/scripts/build.scm (set-build-options-from-command-line):
Pass #:multiplexed-build-output?.
(%default-options): Add 'multiplexed-build-output?'.
* guix/scripts/environment.scm (%default-options): Likewise.
* guix/scripts/pack.scm (%default-options): Likewise.
* guix/scripts/package.scm (%default-options): Likewise.
* guix/scripts/pull.scm (%default-options): Likewise.
* guix/scripts/system.scm (%default-options): Likewise.