Fix header file usage -- there were many unnecessary header files included in

busybox.h which slowed compiles.  I left only what was needed and then fixed up
all the apps to include their own header files.  I also fixed naming for pwd.h
and grp.h functions.  Tested to compile and run with libc5, glibc, and uClibc.
 -Erik

coreutils/sync.c

@@ -23,6 +23,7 @@
 
 #include "busybox.h"
 #include <stdio.h>
+#include <unistd.h>
 
 extern int sync_main(int argc, char **argv)
 {