pstack_watcher: set a limit on backtrace depth
pstack_watcher: set a limit on backtrace depth

On RHEL 6.8, pstack_watcher-test times out fairly reliably, with a 'gdb'
process left spinning 100% CPU.

Some googling found a report of a similar spinning GDB while taking a
backtrace[1]. This report suggests setting a backtrace limit --
apparently the spinning is due to gdb getting in some infinite loop
trying to walk the stack with insufficient/incorrect debug info.

This patch sets such a limit. I verified on a box which was reliably
reproducing the timeout that it is now fixed.

[1] https://bugs.launchpad.net/ubuntu/+source/gdb/+bug/434168

Change-Id: Ide46209256fd4ced6b08065c2ae3c0046de49565
Reviewed-on: http://gerrit.cloudera.org:8080/6658
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Reviewed-by: Mike Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@apache.org>
Tested-by: Kudu Jenkins

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