kwset: use unsigned char to store values with high-bit set

kwset: use unsigned char to store values with high-bit set

Sun Studio on Solaris issues warnings about improper initialization
values being used when defining tolower_trans_tbl[] in ctype.c.  The
array wants to store values with high-bit set and treat them as
values between 128 to 255.  Unlike the rest of the Git codebase
where we explicitly specify 'unsigned char' for such variables and
arrays, however, kwset code we borrowed from elsewhere uses 'char'
for this and other variables.

Fix the declarations to explicitly use 'unsigned char' where
necessary to bring it in line with the rest of the Git.

Signed-off-by: Ben Walton <bdwalton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>