Further reduce spam score by adding encoding info.

A lot of spam filters are more likely to flag email without an encoding in the Content-Type headers as spam. Netdata is technically not locale aware, but the email message is sent from a bash script, and bash is locale aware, so the message will be encoded in whatever the environment dictated character encoding is (which is usually UTF-8) these days. It turns out that my assessment in the previous commit was incorrect, and it actually is trivial to get a MIME compatible encoding name from a shell script, so lets try to make sure the encoding is set automatically. When not explicitly set by the user, we first try to determine the system encoding using the `locale` command. If this fails and the user has not explicitly set an encoding, then it defaults to UTF-8, which is the de-facto standard on all modern Linux distributions and thus should be correct most of the time. Users can override the detection by setting EMAIL_CHARSET in health_alarm_notify.conf. For English speaking users, this should result in no change. For non-English speaking users, this may fix display of non-punycode encoded hostnames.