flux-job attach: limit size of each stdin entry to max size

[chu11]

Per discussion in #4572

Problem: we do not want to overflow the size of the stdin buffer, but its possible a single stdin entry could be larger than the stdin buffer size.

to make things easier, limit each stdin entry to be a max of the stdin buffer size. some simple math around flux job attach's attach_send_shell() should do the trick (did not analyze beyond this cursory analysis).

side note: libsubprocess needs to expose the default stdin buffersize when tools don't allow config of buffer size.

side note: other places to do this?