Avoid some C++ diagnostics reported by Bruno Haible.

Avoid some C++ diagnostics reported by Bruno Haible.
* lib/quotearg.c (clone_quoting_options): Use xmemdup rather than
xmalloc.
(quotearg_alloc): Use xcharalloc rather than xmalloc.
(struct slotvec): Move to top level.
(quotearg_n_options): Rewrite to avoid xmalloc.
* lib/xalloc.h (xcharalloc): New function.
* (xrealloc, xnrealloc, x2realloc, x2nrealloc, xmemdup):
[defined __cplusplus]: Add function template that provides result
type propagation.  This part of the change is from Bruno Haible.