Make Ctrl-C clear line and re-prompt in REPL instead of exiting

Use sigsetjmp/siglongjmp to catch SIGINT during readline/fgets input
and jump back to the REPL loop with a fresh prompt. Running Lua code
still gets interrupted via the existing laction/lstop mechanism.

Fixes #11.

issues/11-ctrl-c-repl-behaviour.md

@@ -1,6 +1,6 @@
 # Issue #11 — Ctrl-C REPL behaviour
 
-**Status:** open
+**Status:** resolved
 
 ## Problem