fixed crash on 32-bit OS with 64-bit time_t.

git-svn-id: svn://sylpheed.sraoss.jp/sylpheed/trunk@3405 ee746299-78ed-0310-b773-934348b2243d
made mbox locking NFS-safe (#202).

git-svn-id: svn://sylpheed.sraoss.jp/sylpheed/trunk@3407 ee746299-78ed-0310-b773-934348b2243d