Console.Unix: fix missing terminal configuration on SIGINT/SIGQUIT/SIGCONT. (#64200)

The introduction of the managed API for signal handling (PosixSignal)
inadvertently caused terminal configuration to no longer be performed
when no managed handlers are registered.

This adds back the unconditional registration for SIGINT/SIGQUIT/SIGCONT.

The missing registrations can cause the terminal to stop echoing when an
application terminates on Ctrl-C.

Author: tmds

src/native/libs/System.Native/pal_signal.c

@@ -584,6 +584,16 @@ int32_t InitializeSignalHandlingCore()
         return 0;
     }
 
+#ifdef HAS_CONSOLE_SIGNALS
+    // Unconditionally register signals for terminal configuration.
+    bool installed = InstallSignalHandler(SIGINT, SA_RESTART);
+    assert(installed);
+    installed = InstallSignalHandler(SIGQUIT, SA_RESTART);
+    assert(installed);
+    installed = InstallSignalHandler(SIGCONT, SA_RESTART);
+    assert(installed);
+#endif
+
     return 1;
 }
 
@@ -612,7 +622,12 @@ void SystemNative_DisablePosixSignalHandling(int signalCode)
     {
         g_hasPosixSignalRegistrations[signalCode - 1] = false;
 
-        if (!(g_consoleTtouHandler && signalCode == SIGTTOU) &&
+        // Don't restore handler when something other than posix handling needs the signal.
+        if (
+#ifdef HAS_CONSOLE_SIGNALS
+            signalCode != SIGINT && signalCode != SIGQUIT && signalCode != SIGCONT &&
+#endif
+            !(g_consoleTtouHandler && signalCode == SIGTTOU) &&
             !(g_sigChldCallback && signalCode == SIGCHLD) &&
             !(g_terminalInvalidationCallback && (signalCode == SIGCONT ||
                                                  signalCode == SIGCHLD ||