Add complevel hints for Default compatibility level (#488)

* Add complevel hints for Default compatibility level

* Explain bizarre spacing

* Revert spacing changes in favor of shorter complevel descriptions

* Remove extra slash from 1.9 complevel

* Abbreviate 'PrBoom' in complevel 12 description

* Consistently abbreviate each instance of PrBoom

prboom2/src/m_menu.c

@@ -3300,25 +3300,25 @@ static const char *gen_skillstrings[] = {
 
 static const char *gen_compstrings[] =
 {
-  "Default",
-  "Doom v1.2",
-  "Doom v1.666",
-  "Doom/2 v1.9",
-  "Ultimate Doom",
-  "Final Doom",
-  "DosDoom",
-  "TASDoom",
-  "Boom's vanilla",
-  "Boom v2.01",
-  "Boom",
-  "LxDoom",
-  "MBF",
-  "PrBoom 2.03b",
-  "PrBoom 2.1.x",
-  "PrBoom 2.2.x",
-  "PrBoom 2.3.x",
-  "PrBoom 2.4.0",
-  "Latest PrBoom+",
+  "17/Default",
+  "0/Doom v1.2",
+  "1/Doom v1.666",
+  "2/Doom (2) v1.9",
+  "3/Ultimate v1.9",
+  "4/Final Doom",
+  "5/DosDoom",
+  "6/TASDoom",
+  "7/Boom vanilla",
+  "8/Boom v2.01",
+  "9/Boom",
+  "10/LxDoom",
+  "11/MBF",
+  "12/PrB 2.03b",
+  "13/PrB 2.1.x",
+  "14/PrB 2.2.x",
+  "15/PrB 2.3.x",
+  "16/PrB 2.4.0",
+  "17/PrB+ Latest",
   NULL
 };