Make FileWatching.Pidfile.isvalidpid public?

I'm using it but seems internal: ```julia julia> using FileWatching julia> Base.isdefined(FileWatching.Pidfile, :isvalidpid) true julia> Base.isexported(FileWatching.Pidfile, :isvalidpid) false julia> Base.ispublic(FileWatching.Pidfile, :isvalidpid) false ```