cpu-set-util: introduce config_parse_cpu_set()

Then, make parse_cpu_set() as a tiny wrapper of it.

Note, previously when an invalid CPU range, e.g. "3-0", is specified,
we ignore the range but allocate an empty set. But, with this commit,
now the conf parser simply ignore it without no side effect.
This potentially changes behavior of a system with such invalid setting,
but the change should be favorable for consistency with other parsers.

src/core/main.c

@@ -741,7 +741,7 @@ static int parse_config_file(void) {
                 { "Manager", "CrashAction",                  config_parse_crash_action,          0,                        &arg_crash_action                 },
                 { "Manager", "ShowStatus",                   config_parse_show_status,           0,                        &arg_show_status                  },
                 { "Manager", "StatusUnitFormat",             config_parse_status_unit_format,    0,                        &arg_status_unit_format           },
-                { "Manager", "CPUAffinity",                  config_parse_cpu_affinity2,         0,                        &arg_cpu_affinity                 },
+                { "Manager", "CPUAffinity",                  config_parse_cpu_set,               0,                        &arg_cpu_affinity                 },
                 { "Manager", "NUMAPolicy",                   config_parse_numa_policy,           0,                        &arg_numa_policy.type             },
                 { "Manager", "NUMAMask",                     config_parse_numa_mask,             0,                        &arg_numa_policy.nodes            },
                 { "Manager", "JoinControllers",              config_parse_warn_compat,           DISABLED_LEGACY,          NULL                              },