Use qemu_tolower() and qemu_toupper(), not tolower() and toupper()

On NetBSD, where tolower() and toupper() are implemented using an
array lookup, the compiler warns if you pass a plain 'char'
to these functions:

gdbstub.c:914:13: warning: array subscript has type 'char'

This reflects the fact that toupper() and tolower() give
undefined behaviour if they are passed a value that isn't
a valid 'unsigned char' or EOF.

We have qemu_tolower() and qemu_toupper() to avoid this problem;
use them.

(The use in scsi-generic.c does not trigger the warning because
it passes a uint8_t; we switch it anyway, for consistency.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com> for the s390 part.
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-id: 1500568290-7966-1-git-send-email-peter.maydell@linaro.org
