Allow resetting of jq err callback

This will be useful for the upcoming test-erroneous-programs improvement
to --run-tests, so we can switch between the default error reporting
method (print to stderr) to a method internal to --run-tests, and back.

The idea is that when testing programs that are expected to compile (and
link), it'd be nice if errors continue going to stderr, while when
testing programs that must fail to compile (or link), the error has to
be captured so it can be compared to the error expected by the test.
Make --run-tests more informative

Print the line number and program text on the same output line where a
failure is reported, for all failures that were already reported on
lines starting with '***'.  This makes poring over test logs with
failures much easier.
Add support for testing erroneous programs

This will allow moving tests of some tests from tests/run to
tests/all.test.  It will also be useful for more extensive testing of
language features where we want nice error messages, or errors at all,
to be produced, particularly language features where the errors arise at
compile-, codegen-, or link-time rather than parse-time.

This does not check for run-time errors though -- those we can already
check for with try/catch.

    $ jq --run-tests
    %%FAIL
    break
    jq: error: break used outside labeled control structure
    ^D
    1 of 1 tests passed (0 malformed)
    $
Fix lineno counting in jq_test.c
Fix parser leak (fuzzing)
Remove string indexing by string (fix #454)

This turns out to have been a bad idea:

    "foo"|.["o"]

it interacts badly with `path()`.

See #454 for the gory details.
Add `label $name | EXP`; fix `break`

This is to fix the problem where `break` is dynamic, not lexical.

With this it should be possible to do this sort of thing:

    label $break | inputs | if ... then $break|error else . end

This is a backwards-incompatible change for master, but the previous
`break` hadn't shipped yet.

Still needed:

 - testing