array: Warn about caveat with array_remove_at() and value based arrays

Because enumerate() for value based arrays returns a pointer directly to
the internal array elements and because array_remove_at() or rather the
called array_remove() may move elements over the element at the currently
enumerated position, the pointer passed to enumerate() will point to a
different array element after the array_remove_at() call.  The caller
will thus operate on the wrong element if that pointer is accessed again
before calling enumerate().

For performance reasons we currently don't change the implementation to copy
each array element during enumeration to a private member of the enumerator and
return a pointer to that.  Similarly, due to the danger of subtle bugs we don't
remember the pointer passed to enumerate() to later redirect it to a copy
created during the array_remove_at() call.
array: Adjust negative index before calling remove_head|tail()

For ARRAY_TAIL we most often want to call remove_tail() not remove_head().
auth-cfg: Fix crash after several reauthentications with multiple authentication rounds

Due to the issue described in c641974, purge() inadvertently destroyed
CA certificates that should have been kept (while the pointer to these
objects remained in the array).  This lead to incorrect reference counts
and after a few reauthentications with multiple authentication rounds,
which cause calls to purge(TRUE), to crashes.