Extract reclaim_tty into reusable function

Move the inline TTY reclamation logic from build.func into a standalone reclaim_tty() function in core.func. This improves code reusability, as the function now needs to be called before each interactive prompt (not just after install) since pct exec/pull/enter can take the terminal away again. The function also includes better error handling with checks for perl availability and /dev/tty existence, and explicitly sets SIGTTOU to IGNORE in perl.

misc/error_handler.func

@@ -499,6 +499,7 @@ error_handler() {
 
       local response=""
       local read_rc
+      declare -f reclaim_tty >/dev/null 2>&1 && reclaim_tty
       read -t 60 -r response </dev/tty
       read_rc=$?
       if [[ $read_rc -eq 0 ]]; then