Enable HTTPS on Linux, through naett's libcurl backend

naett has had a complete libcurl backend all along; we just never built it,
so Linux ran with HTTPS_NOT_AVAILABLE. That means no homebrew store over
HTTPS, and RetroAchievements talking to plain http://retroachievements.org.

libcurl is loaded with dlopen rather than linked, the same way we handle the
Vulkan loader, so it stays a soft dependency: we need the curl headers at
build time, but a build made here still starts on a machine without libcurl
installed - it just reports HTTPS as unavailable, exactly like today. Distro
packagers get the behavior they'd expect either way, and certificate
validation comes free from the system CA store.

New net::HTTPSAvailable() answers "did that work", and SDLMain folds it into
SYSPROP_SUPPORTS_HTTPS, which everything downstream already degrades on.

Four fixes to the backend itself, all noted in ext/naett/README-ppsspp.md:

- panic() called exit(1) on a pipe or curl_multi_perform failure. Taking the
  emulator down because a download failed isn't acceptable - the backend now
  disables itself and requests complete with naettGenericError.
- CURLINFO_RESPONSE_CODE writes a long into res->code, which is an int. Eight
  bytes into four, getting away with it only because the next field absorbs
  the zeroes.
- curl_easy_setopt is varargs and wants a long for these options; int literals
  and int variables are UB on LP64.
- naettPlatformCloseResponse called through a null function pointer when
  libcurl was missing. Found by testing that path, which segfaulted.

CI needs libcurl4-openssl-dev (curl-dev on Alpine) or it would quietly keep
building without HTTPS.

Common/Net/Resolve.cpp

@@ -16,6 +16,12 @@
 
 #ifndef HTTPS_NOT_AVAILABLE
 #include "ext/naett-lib/naett.h"
+// Note: PPSSPP_PLATFORM(LINUX) is also set on Android, which needs no loader.
+#if PPSSPP_PLATFORM(LINUX) && !PPSSPP_PLATFORM(ANDROID)
+// On Linux, naett goes through libcurl, which we load at runtime - so HTTPS support is
+// only known once we've tried.
+#include "ext/naett-lib/src/naett_curl.h"
+#endif
 #endif
 
 #if PPSSPP_PLATFORM(ANDROID)
@@ -44,13 +50,25 @@ void Init() {
 		_assert_(gJvm != nullptr);
 		naettInit(gJvm);
 #else
-		naettInit(NULL);
+		if (HTTPSAvailable()) {
+			naettInit(NULL);
+		}
 #endif
 #endif
 		g_naettInitialized = true;
 	}
 }
 
+bool HTTPSAvailable() {
+#ifdef HTTPS_NOT_AVAILABLE
+	return false;
+#elif PPSSPP_PLATFORM(LINUX) && !PPSSPP_PLATFORM(ANDROID)
+	return naettCurlLoad() != 0;
+#else
+	return true;
+#endif
+}
+
 void Shutdown() {
 #ifdef _WIN32
 	if (g_wsaInitialized) {