cmd/dirserver: handle links
cmd/dirserver: handle links

This is straightforward, just need to be careful about when a DirEntry
can be nil and to return the non-nil entry on ErrFollowLink.

Change-Id: I962ef76cb6317ca951805d8dcc0b528ee0801955
Reviewed-on: https://upspin-review.googlesource.com/4422
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

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