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
diff --git a/df.c b/df.c
index 22797fb..0408f9f 100644
--- a/df.c
+++ b/df.c
@@ -24,8 +24,10 @@
 
 #include "busybox.h"
 #include <stdio.h>
+#include <stdlib.h>
 #include <mntent.h>
 #include <sys/vfs.h>
+#include <getopt.h>
 
 extern const char mtab_file[];	/* Defined in utility.c */
 #ifdef BB_FEATURE_HUMAN_READABLE