Add unsafe option to notify for unsafe unblocking

[IanButterworth]

In trying to find ways to unblock readline(stdin) for implementing a timeout for Base.prompt #39027, adding an unsafe option to notify to disable concurrency violation checks, and temporarily setting the io to closed seemingly "works" without breaking the repl stdin, but I suspect it's not legal?

julia> function readline_timeout(io::Base.TTY, timeout)
           stat_before = io.status
           Timer(timeout) do t
               io.status = Base.StatusClosed
               notify(io.cond, unsafe=true)
           end
           try
               readline(io)
           finally
               io.status = stat_before
          end
       end
readline_timeout (generic function with 1 method)

julia> readline_timeout(stdin, 3)
""

julia> versioninfo()
Julia Version 1.7.0-DEV.864
Commit d02aba4c21* (2021-04-07 16:11 UTC)
...

[vtjnash]

There is no point in having a lock that is unsafe to use

[IanButterworth]

Ok. Am I right in saying that readline being non-unblockable is a design choice? So I should quit trying to work around it?

[IanButterworth]

Scratch that, just saw your suggestion in #39027 (comment)

I'll give that a go. Thanks