Mount attach message clean-up

[dsheets]

This patchset fixes some issues with the mount-time attach message handling. It calls the callback's attach message so that the file system can perform actions consistently on all attach message including the first. There are also a number of other minor improvements.

[djs55]

The tests fail for me locally too, with:

[OK]        response 13   print then parse ((tag (14)) (payload (Wstat ()))).
Test Successful in 0.004s. 27 tests run.
[OK]        client  0   connect1.
[OK]        client  1   connect2.
[ERROR]     client  2   check that create rebinds fids.
[ERROR]     client  3   check that we can remove a file.
[ERROR]     client  4   check that we can remove a directory.
[ERROR]     client  5   failed remove should clunk the fid.
 ...        client  6   check reading out-of-bounds on a directory doesn't fail [ERROR]     client  6   check reading out-of-bounds on a directory doesn't fail badly.
5 errors! in 0.022s. 7 tests run.
W: Test 'test_client_lofs_server' fails: Command '/Users/djs/djs55/ocaml-9p/_build/lib_test/lofs_test.native' terminated with error code 5
E: Failure("Tests had a 50.00% failure rate")
make: *** [test] Error 1

I can take a look in an hour or two.

[dsheets]

The light is green and the trap is clean.

[djs55]

The Merge is made!