tree-wide: check if errno is greater then zero

gcc is confused by the common idiom of
  return errno ? -errno : -ESOMETHING
and thinks a positive value may be returned. Replace this condition
with errno > 0 to help gcc and avoid many spurious warnings. I filed
a gcc rfe a long time ago, but it hard to say if it will ever be
implemented [1].

Both conventions were used in the codebase, this change makes things
more consistent. This is a follow up to bcb161b023.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61846

src/core/execute.c

@@ -2319,7 +2319,7 @@ int exec_context_load_environment(Unit *unit, const ExecContext *c, char ***l) {
                                 continue;
 
                         strv_free(r);
-                        return errno ? -errno : -EINVAL;
+                        return errno > 0 ? -errno : -EINVAL;
                 }
                 count = pglob.gl_pathc;
                 if (count == 0) {