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/README.txt b/minadbd/README.txt
index 0c190d0..c9df484 100644
--- a/minadbd/README.txt
+++ b/minadbd/README.txt
@@ -4,16 +4,19 @@
 adb.c
   - much support for host mode and non-linux OS's stripped out; this
     version only runs as adbd on the device.
-  - does not setuid/setgid itself (always stays root)
+  - always setuid/setgid's itself to the shell user
   - only uses USB transport
   - references to JDWP removed
   - main() removed
+  - all ADB_HOST and win32 code removed
+  - removed listeners, logging code, background server (for host)
 
 adb.h
   - minor changes to match adb.c changes
 
 sockets.c
   - references to JDWP removed
+  - ADB_HOST code removed
 
 services.c
   - all services except echo_service (which is commented out) removed
@@ -25,3 +28,12 @@
 Android.mk
   - only builds in adbd mode; builds as static library instead of a
     standalone executable.
+
+sysdeps.h
+  - changes adb_creat() to use O_NOFOLLOW
+
+transport.c
+  - removed ADB_HOST code
+
+transport_usb.c
+  - removed ADB_HOST code