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/expr.c

@@ -35,9 +35,11 @@
 
 #include "busybox.h"
 #include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <regex.h>
 #include <sys/types.h>
 
-#include <regex.h>
 
 /* The kinds of value we can have.  */
 enum valtype {