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.

.github/workflows/build.yml

@@ -321,7 +321,7 @@ jobs:
       if: runner.os == 'Linux' && matrix.extra != 'android' && matrix.extra != 'loongarch64'
       run: |
         sudo apt-get update -y -qq
-        sudo apt-get install libsdl3-dev libgl1-mesa-dev libglu1-mesa-dev libsdl3-ttf-dev libfontconfig1-dev
+        sudo apt-get install libsdl3-dev libgl1-mesa-dev libglu1-mesa-dev libsdl3-ttf-dev libfontconfig1-dev libcurl4-openssl-dev
 
     - name: Install macOS SDL3 dependencies
       if: runner.os == 'macOS' && matrix.id == 'macos'
@@ -475,7 +475,7 @@ jobs:
       if: runner.os == 'Linux'
       run: |
         sudo apt-get update -y -qq
-        sudo apt-get install libsdl3-dev libgl1-mesa-dev libglu1-mesa-dev libsdl3-ttf-dev libfontconfig1-dev
+        sudo apt-get install libsdl3-dev libgl1-mesa-dev libglu1-mesa-dev libsdl3-ttf-dev libfontconfig1-dev libcurl4-openssl-dev
 
     - name: Install macOS dependencies
       if: runner.os == 'macOS'
@@ -524,7 +524,7 @@ jobs:
     steps:
     - name: Install Linux dependencies (Alpine)
       run: |
-        apk add build-base wget git bash cmake python3 glu-dev sdl3-dev sdl3_ttf-dev
+        apk add build-base wget git bash cmake python3 glu-dev sdl3-dev sdl3_ttf-dev curl-dev
 
     - uses: actions/checkout@v7
       with: