add make genpath rules, .path files, updated .top and .year

Add .path files to each winner directory: a file containing
the path to the winner directory from the top directory.

Changed YYYY/.year files to contain the paths, from the top
directory, if the winning directories for the given year.

Changed `/.top` to contain the IOCCC years.

Added `make genpath` rule to the top Makefile and the
year Makefiles to generate the above.

Added FAQ 6.8 to `faq.md` to explain the above.

Updated manifest to reflect recent changes to the list of files.

2013/.year

@@ -1 +1,15 @@
-2013
+2013/birken
+2013/cable1
+2013/cable2
+2013/cable3
+2013/dlowe
+2013/endoh1
+2013/endoh2
+2013/endoh3
+2013/endoh4
+2013/hou
+2013/mills
+2013/misaka
+2013/morgan1
+2013/morgan2
+2013/robison