CACHEFILE specifies how the file name of a cache file is formed.
The name is derived mainly from informations about server host and
Those informations are specified in the format "$[server:format]"
where the format is a sequence of symbols each refers a part of an URLscheme://host.d2.d1:port/path
Another formatting pattern is "$[hash:format]" which
hashes a string generated by format into
hexadecimal value ranging from "00" to "1f".
This will be useful to divide a single huge directory containing all
servers into 32 small directories, which can be on physically