If ALWASY_REALLOC is defined and we are to 'shrink' memory block,
memcpy() will write outside memory just allocated.

Signed-off-by: Andrew Tridgell <tridge@samba.org>
(cherry picked from commit 8efabcc8a5dcd83deed8ef8e17826a1d347e6d83)

When we disable null tracking, we need to move any existing objects
that are under the null_context to be parented by the true NULL
context.

We also need a new talloc_enable_null_tracking_no_autofree() function,
as the talloc testsuite cannot cope with the moving of the autofree
context under the null_context as it wants to check exact counts of
objects under the null_context, and smbtorture has a large number of
objects in the autofree_context from .init functions

It's annoying when you use 
  p talloc_report_full(ctx, fopen("/tmp/xx","w"))
in gdb, and if you don't have write permission on the file then
you get a segv.

If NULL tracking is enabled after the autofree context is initialised
then autofree ends up separate from the null_context. This means that
talloc_report_full() doesn't report the autofree context. Fix this by
reparenting the autofree context when we create the null_context.

When we get a double free abort from talloc it is often hard to work
out where the first free came from. This patch takes advantage of the
fact that _talloc_free() now takes a location the free was called from
to allow the double free abort code to print the location of the first
free that conflicts.

We also use the major and minor versions in the TALLOC_MAGIC,
so that we can detect if two conflicting versions of talloc
are loaded in one process. In this case we use talloc_log() to
output a very useful debug message before we call
talloc_abort().

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So that the application can setup a log function to get ERROR
and WARNING messages.

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We have to many callers, which rely on that talloc_steal() never fails.

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