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

uptime.c

@@ -32,6 +32,7 @@
 #include <stdio.h>
 #include <time.h>
 #include <errno.h>
+#include <stdlib.h>
 
 static const int FSHIFT = 16;              /* nr of bits of precision */
 #define FIXED_1         (1<<FSHIFT)     /* 1.0 as fixed-point */