Minor updates to several README.md files

Just some formatting fixes and in some cases add links to XQuartz for
macOS Mountain Lion and beyond. More entries (probably many more) have
to have that link added but those will be found and fixed in time.

1988/isaak/README.md

@@ -11,11 +11,11 @@
 
         make all
 
-### To run
+## To run:
 
 	./isaak
 
-### Alternate code
+### Alternate code:
 
 An alternate version of this entry, `isaak.alt.c`.