[shttpd] Add a few select MIME types
[shttpd] Switch default MIME type to application/octet-stream
[shttpd] Use last "."-separated path component as extension
[shttpd] Trim indentation from the error template
[shttpd] Send content-length header
[shttpd] Manually send body data in 32'000 byte chunks

This is only necessary to work around several bugs (e.g. cc#2098 &
cc#2160) in R3's networking stack and R3's internal chunking. Once thse
are fixed, we should switch back to directly sending the whole body at
once.