* srfi-13.c (scm_string_trim, scm_string_trim_right,
        scm_string_trim_both): Cast to unsigned char for isspace.

glibc works with negative char values, but c99 doesn't allow them.

On solaris 7, isspace is an array lookup and a char as an index
provokes a warning from gcc (3.3 at least).

This would probably be for the 1.6 branch too.