SystemServer: Explicitly open /dev/null for services without StdIO

Spawning services with nothing open at all on the standard I/O fds is
way too harsh. We now open /dev/null for them instead.

Base/usr/share/man/man5/SystemServer.md

@@ -18,7 +18,7 @@ describing how to launch and manage this service.
 
 * `Executable` - an executable to spawn. If no explicit executable is specified, SystemServer assumes `/bin/{service name}` (for example, `/bin/WindowServer` for a service named `WindowServer`).
 * `Arguments` - a space-separated list of arguments to pass to the service as `argv` (excluding `argv[0]`). By default, SystemServer does not pass any arguments other than `argv[0]`.
-* `StdIO` - a path to a file to be passed as standard I/O streams to the service. By default, services inherit SystemServer's own standard I/O streams, which are normally set to `/dev/tty0`.
+* `StdIO` - a path to a file to be passed as standard I/O streams to the service. By default, services run with `/dev/null` for standard I/O.
 * `Priority` - the scheduling priority to set for the service, either "low", "normal", or "high". The default is "normal".
 * `KeepAlive` - whether the service should be restarted if it exits or crashes. For lazy services, this means the service will get respawned once a new connection is attempted on their socket after they exit or crash.
 * `Lazy` - whether the service should only get spawned once a client attempts to connect to their socket.

Servers/SystemServer/Service.cpp

@@ -190,6 +190,11 @@ void Service::spawn()
             close(STDIN_FILENO);
             close(STDOUT_FILENO);
             close(STDERR_FILENO);
+
+            int fd = open("/dev/null", O_RDWR);
+            ASSERT(fd == STDIN_FILENO);
+            dup2(STDIN_FILENO, STDOUT_FILENO);
+            dup2(STDIN_FILENO, STDERR_FILENO);
         }
 
         if (!m_socket_path.is_null()) {