xalloc-oversized: new module

xalloc-oversized: new module

Due to inline functions, mere inclusion of xalloc.h can result
in a link dependency on xalloc_die() on some platforms.  However,
there are several modules that want to use just xalloc_oversized
in order to short-circuit the potential to call xalloc_die.
Splitting the macro into a new header and module makes this easy.

* modules/xalloc-oversized: New module.
* modules/xalloc (Depends-on): Add it.
* lib/xalloc.h (xalloc_oversized): Move...
* lib/xalloc-oversized.h: ...into new file.

Signed-off-by: Eric Blake <eblake@redhat.com>