cpu-set-util: Accept commas as separators in parse_cpu_set_and_warn

Tested CPUAffinity settings on both a service unit and in system.conf
and confirmed they work as expected.

Added a new test to confirm that trailing commas and spaces work and to
prevent any regressions in that area.

src/basic/cpu-set-util.c

@@ -75,7 +75,7 @@ int parse_cpu_set_and_warn(
                 unsigned cpu;
                 int r;
 
-                r = extract_first_word(&rvalue, &word, WHITESPACE, EXTRACT_QUOTES);
+                r = extract_first_word(&rvalue, &word, WHITESPACE ",", EXTRACT_QUOTES);
                 if (r < 0) {
                         log_syntax(unit, LOG_ERR, filename, line, r, "Invalid value for %s: %s", lvalue, whole_rvalue);
                         return r;