ipn/{ipnserver,ipnlocal}: support incoming Taildrop on Synology

If the user has a "Taildrop" shared folder on startup and
the "tailscale" system user has read/write access to it,
then the user can "tailscale file cp" to their NAS.

Updates #2179 (would be fixes, but not super ideal/easy yet)

Change-Id: I68e59a99064b302abeb6d8cc84f7d2a09f764990
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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