errno is positive

Make sure we compare errno against positive error codes.
The ones in hwclock.c and install.c can have an impact, the
rest are unlikely to be hit or in code that isn't widely
used.

Also check that errno > 0, to help gcc know that we are
returning a negative error code.

src/shared/install.c

@@ -1637,7 +1637,7 @@ UnitFileState unit_file_get_state(
                         return state;
 
                 r = unit_file_can_install(&paths, root_dir, path, true);
-                if (r < 0 && errno != -ENOENT)
+                if (r < 0 && errno != ENOENT)
                         return r;
                 else if (r > 0)
                         return UNIT_FILE_DISABLED;