Hide zsh error messages about 'set -m'

[Ralf Wildenhues]

zsh 4.3.4 will output an error message on stderr when 'set -m' is used,
but not with 'set +m'.  However, the command will exit 0, and my testing
showed that stopping, interrupting, or killing the suite still works
flawlessly with this shell.

Thus applying this patch, which silences errors like these
| ./micro-suite:set:1638: can't change option: -m

and fixes test failures of 155 (parallel test execution) and 160
(parallel autotest and signal handling).

Cheers,
Ralf

        * lib/autotest/general.m4 (AT_JOB_FIFO_FD): Hide zsh 4.3.4
        error messages about `set -m'.

diff --git a/lib/autotest/general.m4 b/lib/autotest/general.m4
index 3462cc0..a1079c8 100644
--- a/lib/autotest/general.m4
+++ b/lib/autotest/general.m4
@@ -1319,7 +1319,7 @@ dnl cause changed test semantics; e.g., a sleep will be 
interrupted.
 dnl Enable job control only for spawning the test group:
 dnl Let the jobs to run in separate process groups, but
 dnl avoid all the status output by the shell.
-    $at_job_control_on
+    $at_job_control_on 2>/dev/null
     (
       # Start one test group.
       $at_job_control_off