patch Issue#44432- proposal for application function hooks

[Vincent Finley <vw...@comcast.net>]

I posted a patch in Issue #44432.  This patch is a proposed feature 
enhancement.

My team's application needs a way to handle memory allocation failures that 
occur inside apr_pool.  For example in a system with tightly managed memory 
limits, malloc can fail.  We need to detect the failure, have a chance to 
adjust memory limits and retry the malloc before APR continues.

This patch provides a way for an application using APR to install simple 
function hooks within APR.  It is a simpler hooks facility than the one found 
in apr_util mainly because it stores hooks statically.

When a memory failure occurs in apr_pool, the hooks code will call the 
function installed by the application.  This the application will have a 
chance to remedy the situation and retry the allocation.

This hooks facility can easily be used in other places where an application 
might need to handle a system failure.

-Vincent Finley