Reserves the share name "magic" for a share whose top-level directory
names encode their own ACL (e.g. "fserb+rhea"). Peers only see and can
descend into top-level dirs that name both the sharer and themselves.
Top-level dir creation/deletion via WebDAV is denied for remote peers;
the directory layout is sharer-local only.
New package drive/magic parses and matches the directory names. The
peerapi handler plumbs the peer's and the local node's tailnet logins
into a new drive.Authz value passed to ServeHTTPWithPerms. The CLI
recognizes the reserved name and validates the path is a directory.
Change-Id: If7b6ad9fdab46b99e7ac5a7c5417a57d61b44478
02631749ba