base: Annotate output with job information.

Before this change, it was difficult to discern which job emitted which
output, as there was no information connecting the job to the output it
produced.  This change rectifies that by annotating each line output by
cron/mcron with a prefix that contains a timestamp and the job name.  It also
reports about when the job runs and whether it completed successfully or
failed.  It was initially suggested here: <https://issues.guix.gnu.org/36510>.

Thanks to the fine people from the #guile libera.chat IRC channel for
providing ideas and help; this change would not have been possible without
them!

* src/mcron/base.scm (install-suspendable-ports!): Install suspendable ports.
(%date-format, %log-format): New parameters.
(validate-date-format, validate-log-format): New procedures.
(<job-data>): New record.
(run-job): Update doc.  Redirect stdout and stderr to a pipe.  Return a
<job-data> instance containing the input port and other information about the
job.  Output job status messages.
(process-output): New procedure.
(child-cleanup): Add docstring.  Use positive logic.  Call 'process-output'
one last time after a child process is collected.
(run-job-loop): Add a CHILDREN-DATA variable to the loop.  Provide the open
file descriptors of the children ports to select*, and collect their output
when they trigger select.
* tests/base.scm ("run-job: basic"): Adjust and fix indentation.
(dummy-job/capture-output): New procedure.
("run-job, output"): New test.
("validate-date-format, valid", "validate-date-format, invalid")
("validate-log-format, valid", "validate-log-format, invalid")
("run-job, output with custom format", "run-job, failure")
("run-job, failure in shell action"): New tests.
* src/mcron/scripts/cron.scm (show-help): Document new options.
(%options) [log-format, date-format]: New options.
(main): Parameterize the main loop with the new parameter options (or their
default values when not provided); move exception handling elsewhere (see
below).
* src/mcron/scripts/mcron.scm: Likewise.
* src/cron.in: Install error handler here.
* src/mcron.in: Likewise.
* doc/mcron.texi: Document new cron and mcron options, as well as new
(mcron base) APIs.
* tests/basic.sh: Test the new options.

Suggested-by: Robert Vollmert <rob@vllmrt.net>

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