run minadbd as shell user

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.

Change-Id: I67b292cf769383f0f67fb934e5a80d408a4c131d
diff --git a/minadbd/README.txt b/minadbd/README.txt
index 0c190d0..1413fe2 100644
--- a/minadbd/README.txt
+++ b/minadbd/README.txt
@@ -4,7 +4,7 @@
 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
@@ -25,3 +25,7 @@
 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
+