[makefile] Add -Wformat-nonliteral as an extra warning category

-Wformat-nonliteral is not enabled by -Wall and needs to be explicitly
 specified.

Modified the few files that use nonliteral format strings to work with
this new setting in place.

Inspired by a patch from Carl Karsten <carl@personnelware.com> and an
identical patch from Rorschach <r0rschach@lavabit.com>.

src/drivers/net/3c595.c

@@ -363,7 +363,7 @@ vxgetlink(void)
         if (n > 0) {
           printf("/");
 	}
-        printf(conn_tab[k].name);
+        printf("%s", conn_tab[k].name );
         n++;
       }
     }

src/drivers/net/forcedeth.c

@@ -452,7 +452,7 @@ static int reg_delay(int offset, u32 mask,
 		delaymax -= delay;
 		if (delaymax < 0) {
 			if (msg)
-				printf(msg);
+				printf("%s", msg);
 			return 1;
 		}
 	} while ((readl(base + offset) & mask) != target);

src/drivers/net/via-rhine.c

@@ -1008,7 +1008,7 @@ rhine_probe1 (struct nic *nic, struct pci_device *pci, int ioaddr, int chip_id,
     unsigned char mode3_reg;
 
     if (rhine_debug > 0 && did_version++ == 0)
-	printf (version);
+	printf ("%s",version);
 
     // get revision id.
     pci_read_config_byte(pci, PCI_REVISION, &revision_id);