keygen: API key generator

A command-line utility to generate an API key for Tinode server

Parameters:

sequence: Sequential number of the API key. This value can be used to reject previously issued keys.
isroot: Currently unused. Intended to designate key of a system administrator.
validate: Key to validate: check previously issued key for validity.
salt: HMAC salt, 32 random bytes base64 encoded or auto to automatically generate salt.