This moves the folder index, text, moderation and tmp files into
directories corresponding to the file name suffixes currently in use.

Old                     New
---                     ---
STATEDIR/foo.i          STATEDIR/index/foo
STATEDIR/foo.i.m        STATEDIR/index/mod/foo
STATEDIR/foo.i.t        STATEDIR/index/new/foo
STATEDIR/foo.t          STATEDIR/text/foo
STATEDIR/foo.t.m        STATEDIR/text/mod/foo
STATEDIR/foo.t.t        STATEDIR/text/new/foo

This has the following advantages:
1. No more ugly folder path building all over the place
2. STATEDIR and PATHSIZE used in far fewer places
3. No longer assumes paths to folder files will fit in 256 chars
4. A tidier STATEDIR
5. Better separation of concerns (more folder stuff hidden in folders.c)
6. Should make it easier to move the folder stuff into the server
7. Improved error checking/reporting

This makes use of the *at family of syscalls so, instead of building
paths, we now use file descriptors.