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 pushed 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.