Added "mail storage conversion" plugin. It can be used with IMAP, POP3 and/or LDA to convert one complete mail storage to another format. Also included a convert-tool command line tool to do it manually. Currently doesn't support preserving UID/UIDVALIDITY.

#ifndef __CONVERT_STORAGE_H
#define __CONVERT_STORAGE_H

int convert_storage(const char *user, const char *home_dir,
		    const char *source_data, const char *dest_data);

#endif