C++ify charsets

This patch makes the charset list an std::vector instead of a VEC.
Because we must have access to the raw pointers as a simple array, we
can't use a vector of unique_ptr/unique_xmalloc_ptr.  Therefore, wrap
the vector in a simple class to facilitate the cleanup.  This allows
removing one usage of free_char_ptr_vec.

gdb/ChangeLog:

	* charset.c (struct charset_vector): New.
	(charsets): Change type to charset_vector.
	(find_charset_names): Adjust.
	(add_one): Adjust.
	(_initialize_charset): Adjust.