Avoid invalid pointer to pointer conversions.

Avoid invalid pointer to pointer conversions.

Casts between 'char **' <-> 'unsigned char **' and 'char **' <-> const
char **' are actually invalid:

  http://gcc.gnu.org/ml/gcc-help/2013-03/msg00118.html

In a nutshell, char (and variants) can alias anything, but pointers to
chars get no special treatment (cf. C99/N1256, 6.5/7).

Turns out older gcc's actually warn/complain on these constructs,
though newer one's don't:

  http://sourceware.org/ml/gdb-patches/2013-03/msg00429.html
  http://sourceware.org/ml/gdb-patches/2013-03/msg00430.html

This patch fixes the cases I added last week.  It also fixes one other
preexisting case in charset.c, though it seems even older gccs don't
complain of char * <-> const char * aliasing.

Tested on x86_64 Fedora 17.

gdb/
2013-03-11  Pedro Alves  <palves@redhat.com>

	* charset.c (convert_between_encodings): Don't cast between
	different pointer to pointer types.  Instead, make the 'inp' local
	be of the type iconv expects.
	(wchar_iterate): Don't cast between different pointer to pointer
	types.  Instead, use new pointer local of the type iconv expects.
	* target.c (target_read_stralloc, target_fileio_read_stralloc):
	Add new local of type char pointer, and use it to get a
	char/string view of the byte buffer, instead of casting between
	pointer to pointer types.