Rename util.c to string.c and introduce string.h.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/hw/ramdisk.c b/src/hw/ramdisk.c
index b9da2ad..6e730bf 100644
--- a/src/hw/ramdisk.c
+++ b/src/hw/ramdisk.c
@@ -10,6 +10,7 @@
 #include "biosvar.h" // GET_GLOBAL
 #include "bregs.h" // struct bregs
 #include "boot.h" // boot_add_floppy
+#include "string.h" // memset
 
 void
 ramdisk_setup(void)