It used to just store received files URL-escaped on disk, but that was
a half done lazy implementation, and pushed the burden to callers to
validate and write things to disk in an unescaped way.
Instead, do all the validation in the receive handler and only
accept filenames that are UTF-8 and in the intersection of valid
names that all platforms support.
Fixestailscale/corp#1594
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
