Increase limited read_stdin() max buffer size
Increase it from 1024 to 4096 characters, which ought to be enough for both passwords and certificates.
Perhaps it would be better to just remove this artificial limit, using dynamic allocation in all cases. For now, I tried to avoid that in the Windows version of the function, which is still using a static buffer.
Fixes #579 (closed).
Edited by Dimitri Papadopoulos Orfanos