Do not use FILE *.

We may be called very early during runtime initialisation, so it does not
seem wise to rely on libc-initialised structures. Instead, only rely on
kernel-provided file descriptors.

userhosts.c

@@ -50,7 +50,7 @@ static inline void *dlsym_or_abort(const char *name) {
   dlerror(); /* Clear any previous error */
   symbol = dlsym(RTLD_NEXT, name);
   if (!symbol && (error = dlerror())) {
-    fprintf(stderr, "Error loading '%s': %s\n", name, error);
+    dprintf(STDERR_FILENO, "Error loading '%s': %s\n", name, error);
     abort();
   }
   return symbol;
@@ -96,7 +96,7 @@ int main(int argc, char *argv[]) {
   int ret = -1;
 
   if (argc <= 1)
-    fprintf(stderr, "usage: userhosts <command> [<args>]\n");
+    dprintf(STDERR_FILENO, "usage: userhosts <command> [<args>]\n");
   else {
     /* In order not to depend on /proc, getauxval(AT_EXECFN) from <sys/auxv.h>
      * should be used. However, this requires glibc >= 2.16. */