`isspace` takes `int` but documents that the value must be representable
by `unsigned char`, or be EOF.  Use a wrapper to cast to `unsigned char`
to avoid sign extension while converting to `int`.  This generalizes the
fix from commit 5e8c176e (cmExecuteProcessCommand: Cast c to unsigned
char before cast to int, 2024-01-05) to other `isspace` call sites.

This was detected by assertions in the MSVC standard library while
processing UTF-8 text.

Issue: #25561