Add sys_vumap() kernel call

This new call is a vectored version of sys_umap(). It supports batch
lookups, non-contiguous memory, faulting in memory, and basic access
checks.

kernel/system.c

@@ -224,6 +224,7 @@ PUBLIC void system_init(void)
   /* Copying. */
   map(SYS_UMAP, do_umap);		/* map virtual to physical address */
   map(SYS_UMAP_REMOTE, do_umap_remote);	/* do_umap for non-caller process */
+  map(SYS_VUMAP, do_vumap);		/* vectored virtual to physical map */
   map(SYS_VIRCOPY, do_vircopy); 	/* use pure virtual addressing */
   map(SYS_PHYSCOPY, do_copy);	 	/* use physical addressing */
   map(SYS_SAFECOPYFROM, do_safecopy_from);/* copy with pre-granted permission */