tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>
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