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The root of all filesystems should be a directory node
except in the special case of blob URL where we maintain
the legacy behavior of having the mount point be a file
containing the blob content.

Also, fix the Access() call for http filesystem so that
it honors the access bits of the node itself.

This fixes an issue in naclports where a mount() of
an http filsystem followed by a chdir(http_root) would
always fail with ENOTDIR.

TEST=nacl_io_test

Review URL: https://codereview.chromium.org/547713002

Cr-Commit-Position: refs/heads/master@{#294000}