This is a followup of gnolang#978. Instead of starting in multi-line mode prior
to submit, the line is parsed, and new inputs are appended to it as long
as the statment is not complete, as detected by the Go scanner.

This is simpler and more general than previous attempt. The secondary
prompt is "...", different from primary "gno>", similarly to many REPL
programs (node, python, bash, ...).

The "/editor" command is removed as not useful anymore. Note also
that it is now possible to exit using Ctrl-D.

…ions from Morgan