run minadbd as shell user, remove unused code

Make minadbd drop its root privileges after initializing.  We need to
make the /tmp directory writable by the shell group so that it can
drop the sideloaded file there.
diff --git a/minadbd/services.c b/minadbd/services.c
index 8fc8b3c..aef37f7 100644
--- a/minadbd/services.c
+++ b/minadbd/services.c
@@ -53,6 +53,7 @@
 
     fd = adb_creat(ADB_SIDELOAD_FILENAME, 0644);
     if(fd < 0) {
+        fprintf(stderr, "failed to create %s\n", ADB_SIDELOAD_FILENAME);
         adb_close(s);
         return;
     }