Update rcube_utils::parse_host, fixes #6746

Updated regexps used in parse_host to ensure that %t, %d, %z do not cut off domain and return only tld when underlying host has no subdomain (i.e., is just domain.tld rather than mail.domain.tld). Update fixes #6746, now returns nothing shorter than domain.tld.

Also removed backslash from character class, period does not need to be escaped within character class.