Big cleanup of the use of regular and const string pointers. Where possible, const is used to enfore R/O-ness of string arguments.

Important change to the NVR code to "fix" the PS/1 and PS/2 systems, plus a cleanup.
All device tables now end with a NULL, not an empty string.
Configuration code now deals better with this.
Machine and Video lookup now return -1 if not found/available, and pc_init() will throw the user into Setup if that is the case.
If for some reason the configured memory size exceeds machine limit, it is now adjusted.

src/mem.c

@@ -1569,12 +1569,12 @@ mem_reset(void)
      * exceed the physical limit of the machine to avoid
      * nasty crashes all over the place.
      */
+    m = mem_size;
     c = machines[machine].max_ram;
     if (AT)
 	c <<= 10;	/* make KB */
-    if (mem_size > c) {
-	pclog("MEM: %luKB exceeds machine limit (%luKB), adjusted!\n",
-							mem_size, c);
+    if (m > c) {
+	pclog("MEM: %luKB exceeds machine limit (%luKB), adjusted!\n", m, c);
 	mem_size = c;
     }