Reimplement `downloadURLToFile` using `distsign.Download` and move all
of the progress reporting logic over there.
Updates #6995
Updates #755
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
To make key management less error-prone, use different PEM block types
for root and signing keys. As a result, separate out most of the Go code
between root/signing keys too.
Updates #8760
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>