Rewrite ap_regexec() without a thread-local storage context for allocations.

Rewrite ap_regexec() without a thread-local storage context for allocations.

Provide custom malloc() and free() implementations that use a stack buffer
for first N bytes and then fall back to an ordinary malloc/free().

The key properties of this approach are:

1) Allocations with PCRE2 happen the same way as they were happening
with PCRE1 in httpd 2.4.52 and earlier.

2) There are no malloc()/free() calls for typical cases where the
match data can be kept on stack.

3) The patch avoids a malloc() for the match_data structure itself,
because the match data is allocated with the provided custom malloc()
function.

4) Using custom allocation functions should ensure that PCRE is not
going to use malloc() for any auxiliary allocations, if they are
necessary.

5) There is no per-thread state.

References:
1) https://lists.apache.org/thread/l6m7dqjkk0yy3tooyd2so0rb20jmtpwd
2) https://lists.apache.org/thread/5k9y264whn4f1ll35tvl2164dz0wphvy

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1902572 13f79535-47bb-0310-9956-ffa450edef68