Remove flush note retry loop

The retry loop was intended to make sure that we flush the
process, but the approach is very brittle. For one, user none
isn't allowed to send notes at all which would cause this
loop to get stuck forever.

For now, flushing isn't going to work at all if you run unionfs
under 'none'.

Implement Tflush handling

This patch implements Tflush message handling for Topen,
Tread, and Twrite requests.  These are the most common things
you'd happen to flush, but it's worth noting that unionfs,
being a proxying server, does potentially blocking system
calls while handling most of the requests.  At some point I'll
add that, perhaps when I figure out some more imaginative way
of doing it.

Fix relative branch paths

Release mountpoint as soon as possible

Forgot to copy Dir.length, oops

Make error checks consistent

Close fd on error

Don't sysfatal on bind error

Use /mnt/exportfs mntgen

Remove realpath instead of path

Implement deduplication of directory entries

Fix String shenanigans

Use Strings for paths

This reduces the amount of obnoxious allocations and copying.

Fix merge

merge

Rewrite and simplify the whole lot

Directory reads are done using union binds which allows
removal of a bunch of code.
Fid state management is simplified. Instead of refcounting
clones etc. we just copy everything, which makes things
easier to reason about and removes the need to introduce
locking for FILE structs.
Various other minor improvements are included as well.

Forgottten the qmap.c file...

Simplify branch code, improve naming

Improve Qid mapping strategy and implementation

Remove another unnecessary check