stdlib: Support malloc-managed environ arrays for compatibility

stdlib: Support malloc-managed environ arrays for compatibility

Some applications set environ to a heap-allocated pointer, call
setenv (expecting it to call realloc), free environ, and then
restore the original environ pointer.  This breaks after
commit 7a61e7f557a97ab597d6fca5e2d1f13f65685c61 ("stdlib: Make
getenv thread-safe in more cases") because after the setenv call,
the environ pointer does not point to the start of a heap allocation.
Instead, setenv creates a separate allocation and changes environ
to point into that.  This means that the free call in the application
results in heap corruption.

The interim approach was more compatible with other libcs because
it does not assume that the incoming environ pointer is allocated
as if by malloc (if it was written by the application).  However,
it seems to be more important to stay compatible with previous
glibc version: assume the incoming pointer is heap allocated,
and preserve this property after setenv calls.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>