Split off sys_umap_remote from sys_umap

sys_umap now supports only:
- looking up the physical address of a virtual address in the address space
  of the caller;
- looking up the physical address of a grant for which the caller is the
  grantee.

This is enough for nearly all umap users. The new sys_umap_remote supports
lookups in arbitrary address spaces and grants for arbitrary grantees.

kernel/system.c

@@ -222,6 +222,7 @@ 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_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 */