Better charset handling of outbound mail

The default character set for outbound mail is now auto-determined (when not explicitly specified for a message) between UTF-8, ASCII, and CP437. The [mail] DefaultCharset setting (which fell-back to iso-8859-1 if blank) is no longer "a thing".

Also: specify 8-bit content-transfer-encoding for the (potentially UTF-8 or CP-437) plain text portion of a MIME-encoded message with file attachment (7-bit was wrong) and pass down the text sub-type (e.g. could be "html") for inclusion in the mime-part header (don't assume text/plain, but still use that as default).

Also: log an error when failing to delete an attached file (e.g. from data/file/*.out).