Userland: Always enter jail mode in Browser and Assistant

These programs are capable of running other programs, so we should
restrict them from potentially running SUID programs, which was never a
functionality we supported for those programs anyway.

Userland/Applications/Assistant/main.cpp

@@ -148,6 +148,8 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
     TRY(Core::System::pledge("stdio recvfd sendfd rpath cpath unix proc exec thread map_fixed"));
 
+    TRY(Core::System::enter_jail_mode());
+
     Core::LockFile lockfile("/tmp/lock/assistant.lock");
 
     if (!lockfile.is_held()) {

Userland/Applications/Browser/main.cpp

@@ -180,6 +180,8 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     TRY(Core::System::unveil("/bin/Browser", "x"));
     TRY(Core::System::unveil(nullptr, nullptr));
 
+    TRY(Core::System::enter_jail_mode());
+
     WebView::ChromeProcess chrome_process;
     if (TRY(chrome_process.connect(specified_urls, new_window)) == WebView::ChromeProcess::ProcessDisposition::ExitProcess) {
         outln("Opening in existing process");