VFS namecache, as is done by the Coda module on Linux. Unlike the Coda
namecache, the global VFS namecache isn't tagged by credential, so use
ore conservative flushing behavior (for now) when CODA_PURGEUSER is
issued by Venus.
This improves overall integration with the FreeBSD VFS, including
allowing __getcwd() to work better, procfs/procstat monitoring, and so
on. This improves shell behavior in many cases, and improves ".."
handling. It may lead to some slowdown until we've implemented a
specific access cache, which should net improve performance, but in the
mean time, lookup access control now always goes to Venus, whereas
previously it didn't.
MFC after: 1 month
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