Purge set_eof from IO subsystem.

Being at EOF is something the underlying system handle keeps track of, and not
something we should be able to affect from userspace.
Replace raw_reads from IO_BUFFER with PIO_BF_UNDERFLOW flag
Implement get_bool(), eof(), is_open() in Handle PMC, not its subclasses
Set EOF in io_filehandle_read_b()
Change test to match new get_bool() semantics
Fix bug in Parrot_io_readline_s()

io_verify_has_read_buffer() expects a flags argument, not the buffer size
Consider buffered sockets at EOF if the last read failed
Check for is_eof() in io_readline_encoded_string() instead of the prior hack