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

archival/gzip.c

@@ -45,6 +45,7 @@
  */
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 #define memzero(s, n)     memset ((void *)(s), 0, (n))
 
 #ifndef RETSIGTYPE