Declare (and simplify) uid_from_user.

uid_from_user doesn't need to cope with numeric ids in the BSD code
like it did with the old Android code; the caller now handles those.

Also explain what SUPPORT_DOT is for.

Change-Id: I185c9f02b7039795069aa30545563b8a6ef54cd5
diff --git a/toolbox/Android.mk b/toolbox/Android.mk
index 682ce59..3deb3e7 100644
--- a/toolbox/Android.mk
+++ b/toolbox/Android.mk
@@ -96,7 +96,7 @@
 
 LOCAL_CFLAGS += \
     -Wno-unused-parameter \
-    -DSUPPORT_DOT \
+    -include bsd-compatibility.h \
 
 LOCAL_SHARED_LIBRARIES := \
 	libcutils \