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.

2018/.year

@@ -1 +1,15 @@
-2018
+2018/anderson
+2018/algmyr
+2018/bellard
+2018/burton1
+2018/burton2
+2018/ciura
+2018/endoh1
+2018/endoh2
+2018/ferguson
+2018/giles
+2018/hou
+2018/mills
+2018/poikola
+2018/vokes
+2018/yang