Show job handle in "show unique jobs" output

[p-alik]

Fixes #81, as discussed with @esabol in #81 (comment).

Each line of show unique jobs (gearadmin --show-unique-jobs) is now <job handle>\t<unique> instead of just <unique>. That lets you go from a unique ID to the handle that cancel job / gearadmin --cancel-job needs.

$ gearadmin --show-unique-jobs
H:host:1	myuniq
.

The handle comes first because it never contains a tab, while a unique ID may. Splitting each line on the first tab therefore stays unambiguous, and the layout matches show jobs.

Incompatible output change: scripts that read the whole show unique jobs line as the unique ID need to take the text after the first tab instead. This is targeted at 2.2.0.

Changes:

• libgearman-server/text.cc: print job_handle\tunique per line.
• docs/source/protocol/text.rst, docs/source/bin/gearadmin.rst, gearadmin --help: describe the new output (also fixes the "processesing" typo).
• tests/cli.cc: new test. It submits a background job with a known unique ID to a function that has no worker, checks that --show-unique-jobs prints <handle>\t<unique>, then passes that handle to --cancel-job and checks that --show-jobs reports the job as cancelled (ignore_job=1). The test fails without the server change.

Note: cancel job is lazy. It marks the job as ignored, and the job is freed only when a worker would take it, so a cancelled job keeps showing up in show jobs / show unique jobs until then. That behaviour is unchanged here.

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[esabol]

Thanks, @p-alik !