Fix compilation with GCC 4.4.

Fix compilation with GCC 4.4.

Use std::iostream functions instead of EOF and cl_EOF macros.
This enables us to get rid of the freadchar and funreadchar functions.
While at it, removed the extra signatures introduced for CLN-1.2.0
compatibilty. After all, the soname has been bumped.

Also updated largest known Mersenne prime in examples/perfnum.cc.