Add junk/spam mail folder detection

* src/oomhelp.cpp, src/oomhelp.h (is_junk_mail): New.

We can't access attachments in junk so we need some special
handling. As there is no "Junk" property we do this by
comparing the entryid of the accounts junk folder with
the actual folder of the mail.

• GnuPG-Bug-Id: T4188