MINOR: tools: support an execution context that is just a function

MINOR: tools: support an execution context that is just a function

The purpose here is to be able to spot certain callbacks, such as the
SSL message callbacks, which are difficult to associate to anything.
Thus we introduce a new context type, TH_EX_CTX_FUNC, for which the
context is just the function pointed to by the void *pointer. One
difficulty with callbacks is that the allocation and release contexts
will likely be different, so the code should be properly structured
to allow proper tracking, either by instrumenting all calls, or by
making sure that the free calls are easy to spot in a report.