status: Record more information about builds.

status: Record more information about builds.

* guix/status.scm (<build>): New record type.
(build, matching-build): New procedures.
(compute-status): Adjust to manipulate <build> records instead of
derivation file names in 'build-status-builds-completed' and
'build-status-building'.
(build-event-output-port)[process-line]: Use 'string-split' to preserve
spaces.
* tests/status.scm ("compute-status, builds + substitutes")
("compute-status, missing events"): Adjust to expect <build> records.
Produce complete "build-started" events.
("compute-status, multiplexed build output"): Likewise, and remove
"bar.drv" from 'builds-completed'.