af_unix: Kconfig: make CONFIG_UNIX bool

Let's make CONFIG_UNIX a bool instead of a tristate.
We've decided to do that during discussion about SCM_PIDFD patchset [1].

[1] https://lore.kernel.org/lkml/20230524081933.44dc8bea@kernel.org/

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: Luca Boccassi <bluca@debian.org>
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/Kconfig

@@ -4,7 +4,7 @@
 #
 
 config UNIX
-	tristate "Unix domain sockets"
+	bool "Unix domain sockets"
 	help
 	  If you say Y here, you will include support for Unix domain sockets;
 	  sockets are the standard Unix mechanism for establishing and
@@ -14,10 +14,6 @@ config UNIX
 	  an embedded system or something similar, you therefore definitely
 	  want to say Y here.
 
-	  To compile this driver as a module, choose M here: the module will be
-	  called unix.  Note that several important services won't work
-	  correctly if you say M here and then neglect to load the module.
-
 	  Say Y unless you know what you are doing.
 
 config UNIX_SCM