Raise interrupt exception on Ctrl-C at the read prompt

Raise interrupt exception on Ctrl-C at the read prompt

While rustyline holds the terminal in raw mode, Ctrl-C is delivered as
ReadlineError::Interrupted rather than as a SIGINT, so the ctrlc handler
never sets the INTERRUPT flag. call_readline also lumped Interrupted
into the generic IO-error arm, which read_term then mis-classified as
end_of_file -- so Ctrl-C at a read/1 prompt unified end_of_file instead
of interrupting.

Handle ReadlineError::Interrupted explicitly: set INTERRUPT and return an
Interrupted IO error. read_term now checks the flag in its error arm and
raises the interrupt exception instead of falling through to the EOF or
syntax-error handling.