debugger: `restart` with custom debug port

Fixed a bug in debugger repl where `restart` command did not work
when a custom debug port was specified via command-line option
--port={number}.

File test/simple/helper-debugger-repl.js was extracted
from test/simple/test-debugger-repl.js
doc: Deprecate require.extensions
doc: cluster: s/server.destroy/server.close/

Fixes #5379.