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/firstboot/firstboot.c

@@ -502,7 +502,7 @@ static int write_root_shadow(const char *path, const struct spwd *p) {
 
         errno = 0;
         if (putspent(p, f) != 0)
-                return errno ? -errno : -EIO;
+                return errno > 0 ? -errno : -EIO;
 
         return fflush_and_check(f);
 }