Try harder to match outlooks internal filename enc

* src/common.c (qp_encode): New. Simple quoted printable encoder.
  (b64_encode): Fix termination of return value.
* src/common.h: Add prototype
* src/mimemaker.c (utf8_to_rfc2047b): Use infer content encoding
  to switch between base64 and quoted printable.

As outlook rewrites the attachment filenames in unencrypted mails
we have to guess how they will look so that we sign them correctly.
If it turns out that this is to buggy we should probably replace
non ascii characters in that case.