shared/util: assume ac when /sys/class/power_supply is missing

On s390 (at least) /sys/class/power_supply is not present. We should
treat this like if this directory was empty, and not an error.

(cherry picked from commit 6d89003462)

src/shared/util.c

@@ -5994,7 +5994,7 @@ int on_ac_power(void) {
 
         d = opendir("/sys/class/power_supply");
         if (!d)
-                return -errno;
+                return errno == ENOENT ? true : -errno;
 
         for (;;) {
                 struct dirent *de;