There's a known memory leak in the engine_pkcs11 at the time of writing
this and it interferes with the named ability to check for memory leaks
in the OpenSSL memory context by default.
Add an autoconf option to explicitly enable the memory leak detection,
and use it in the CI except for pkcs11 enabled builds. When this gets
fixed in the engine_pkc11, the option can be enabled by default.
The libxml2 library provides a way to replace the default allocator with
user supplied allocator (malloc, realloc, strdup and free).
Create a memory context specifically for libxml2 to allow tracking the
memory usage that has originated from within libxml2. This will provide
a separate memory context for libxml2 to track the allocations and when
shutting down the application it will check that all libxml2 allocations
were returned to the allocator.
Additionally, move the xmlInitParser() and xmlCleanupParser() calls from
bin/named/main.c to library constructor/destructor in libisc library.
