SSH CheckPeriod does not accept "always"

[phinze]

Tailscale SSH defines the checkPeriod ACL field as either a duration or the string "always".

In our client we are defining the field as just a Duration.

tailscale-client-go/tailscale/client.go

Line 485 in 04b7d00

CheckPeriod Duration `json:"checkPeriod,omitempty" hujson:"CheckPeriod,omitempty"`

tailscale-client-go/v2/policyfile.go

Line 107 in 04b7d00

CheckPeriod Duration `json:"checkPeriod,omitempty" hujson:"CheckPeriod,omitempty"`

This means that we fail to parse policy files that set checkPeriod to "always" with an error like:

time: invalid duration "always"

[bradfitz]

I'd probably just document that negative means always.