ovs-macros.at: Better hide 'exec -a' checking.

ovs-macros.at: Better hide 'exec -a' checking.

There is some issue with parsing of redirection options
on some shells. For example:

  $ (exec -a name true) 2>&1 >/dev/null || echo "failed"
  sh: 10: exec: -a: not found
  failed

  $ (exec -a name true) >/dev/null 2>&1 || echo "failed"
  failed

So, the order of redirections matters for some reason.
Let's replace our current version with simple redirection of stderr.
This version seems to work in most of shells except [t]csh. But it's
really tricky to write portable redirections that works with csh and
this shell will not be used by the testsuite on most of the systems.

With the new version:

  # cat test.sh
  ((exec -a myname true 2>/dev/null) && echo "OK") || echo "fail"

  # sh ./test.sh
  fail
  # bash ./test.sh
  OK
  # tcsh ./test.sh
  -a: Command not found.
  fail

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>