Add compatibility with gmime 2.6

There are lots of API changes in gmime 2.6 crypto handling. By adding
preprocessor directives, it is however possible to add gmime 2.6 compatibility
while preserving compatibility with gmime 2.4 too.

This is mostly based on id:"8762i8hrb9.fsf@bookbinder.fernseed.info".

This was tested against both gmime 2.6.4 and 2.4.31. With gmime 2.4.31, the
crypto tests all work fine (as expected). With gmime 2.6.4, one crypto test is
currently broken (signature verification with signer key unavailable), most
likely because of a bug in gmime which will hopefully be fixed in a future
version.