Mail improvements: - Implement new config option in mail-section "ignore_alternatives" to ignore alternatives in a multipart/alternative mail. The *last* text/plain part of the *first* multipart/alternative is used as the message, if ignore_alternatives is set all other alternative parts of the first multipart/alternative that contained a text/plain part are ignored. Other multipart/alternative or other multipart are attached as before. This fixes [SF#959811] "Multipart/alternative handling considered bad". Note that this also changes which text/plain part is attached as the message if there are several text/plain parts in a multipart: Previously the *first* text/plain would be attached. Now we attach the *last* one, this is more in line with rfc 2046, sec. 5.1.4. according to Philipp Gortan. - Fix bug in attachment of text parts: If there are multiple text/plain parts in a nested multipart, the previous code would attach the multipart serialisation instead of the text/plain serialisation as a file to the issue in some cases. - Add regression tests for the new config-option and bug-fixes above.