fetcher: add /noprompt switch

Deploying WireGuard MSI using Microsoft Endpoint Manager (aka MS Intune)
falls short with poor Microsoft Endpoint Manager support: no ARM64
support, requires multiple per-architecture deployments...

Fetcher proves super-useful for automating WireGuard install. It
contains platform selection logic, MSI download, integrity check...

However, automated installation is an unattended process and the
wireguard-installer.exe must not block the process for any user prompts.

Signed-off-by: Simon Rozman <simon@rozman.si>

installer/fetcher/fetcher.c

@@ -23,7 +23,7 @@
 #include "constants.h"
 
 static char msi_filename[MAX_PATH];
-static volatile bool msi_filename_is_set;
+static volatile bool msi_filename_is_set, prompts = true;
 static volatile size_t g_current, g_total;
 static HWND progress;
 static HANDLE filehandle = INVALID_HANDLE_VALUE;
@@ -208,7 +208,7 @@ static DWORD __stdcall download_thread(void *param)
 	if (security_attributes.lpSecurityDescriptor)
 		LocalFree(security_attributes.lpSecurityDescriptor);
 
-	if (ret) {
+	if (ret && prompts) {
 		ShowWindow(progress, SW_SHOWDEFAULT);
 		if (MessageBoxA(progress, "Something went wrong when downloading the WireGuard installer. Would you like to open your web browser to the MSI download page?", "Download Error", MB_YESNO | MB_ICONWARNING) == IDYES)
 			ShellExecuteA(progress, NULL, "https://" server msi_path, NULL, NULL, SW_SHOWNORMAL);
@@ -285,6 +285,20 @@ static LRESULT CALLBACK wndproc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lPar
 	return DefSubclassProc(hWnd, uMsg, wParam, lParam);
 }
 
+static void parse_command_line(void)
+{
+	LPWSTR *argv;
+	int argc;
+	argv = CommandLineToArgvW(GetCommandLineW(), &argc);
+	if (!argv)
+		return;
+	for (int i = 1; i < argc; ++i) {
+		if (wcsicmp(argv[i], L"/noprompt") == 0)
+			prompts = false;
+	}
+	LocalFree(argv);
+}
+
 int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR pCmdLine, int nCmdShow)
 {
 	MSG msg;
@@ -297,6 +311,8 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR pCmdLine,
 	if (!SetDllDirectoryA("") || !SetDefaultDllDirectories(LOAD_LIBRARY_SEARCH_SYSTEM32))
 		return 1;
 
+	parse_command_line();
+
 	InitCommonControlsEx(&(INITCOMMONCONTROLSEX){ .dwSize = sizeof(INITCOMMONCONTROLSEX), .dwICC = ICC_PROGRESS_CLASS });
 
 	progress = CreateWindowExA(0, PROGRESS_CLASS, "WireGuard Installer",