EggsFS -> TernFS rename

Things not done because probably disruptive:

* kmod filesystem string
* sysctl/debugfs/trace
* metrics names
* xmon instance names

Some of these might be renamed too, but starting with a relatively
safe set.

.gitignore

@@ -13,3 +13,4 @@ kmod/trace
 kmod/vm-out
 linux-*.tar.gz
 eggs-integrationtest.*
+.go-cache