CACHEDIR.TAG - improve code style and the name of the parameter.

Use a macro instead of the global constant `cachedir_tag_signature`.
Use `memcmp` instead of `strncmp`.
Add `has_cachedir_tag` to exclude.h.

(See http://dev.yorhel.nl/ncdu/bug/30)
Let has_cachedir_tag use `malloc` instead of a buffer on the stack.