MINOR: freq_ctr: provide non-blocking read functions

MINOR: freq_ctr: provide non-blocking read functions

Some code called by the debug handlers in the context of a signal handler
accesses to some freq_ctr and occasionally ends up on a locked one from
the same thread that is dumping it. Let's introduce a non-blocking version
that at least allows to return even if the value is in the process of being
updated, it's less problematic than hanging.

(cherry picked from commit 29e246a84ce27af63779d98b305ad53877ae9acc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>