Load / save to libwww-perl (LWP) format files.
Actually, the format is slightly extended from that used by LWP's
(libwww-perl's) HTTP::Cookies, to avoid losing some RFC 2965 information
not recorded by LWP.
It uses the version string "2.0", though really there isn't an LWP Cookies
2.0 format. This indicates that there is extra information in here
(domain_dot and port_spec) while still being compatible with libwww-perl,