CLENAUP: cfgparse-listen: move "email-alert" to its own parser

CLENAUP: cfgparse-listen: move "email-alert" to its own parser

The keyword is now registered in cfg_kws and parsed by
proxy_parse_email_alert(), making it discoverable via "-dKcfg".

The "from", "mailers", "myhostname" and "to" options only differ by the
field they assign, so the field is now selected first and the rest of
the processing, including the check for a missing value, is shared.
~25% code removed in the operation.