vm: add third-party mmap() mode and PROCCTL

these two functions will be used to support all exec() functionality
going into a single library shared by RS and VFS and exec() knowledge
leaving VM.

	. third-party mmap: allow certain processes (VFS, RS) to
	  do mmap() on behalf of another process
	. PROCCTL: used to free and clear a process' address space

etc/system.conf

@@ -11,6 +11,7 @@ service rs
 		RS_SET_PRIV	# 37
 		RS_UPDATE	# 41
 		RS_MEMCTL	# 42
+		PROCCTL
 		;
 	io	NONE;		# No I/O range allowed
 	irq	NONE;		# No IRQ allowed
@@ -93,7 +94,7 @@ service vfs
 		UMAP		# 14
 		VIRCOPY		# 15
 		;
-	vm	BASIC;		# Only basic VM calls allowed
+	vm	PROCCTL;
 	io	NONE;		# No I/O range allowed
 	irq	NONE;		# No IRQ allowed
 	sigmgr          rs;	# Signal manager is RS