configure: build sulogin, watchdogd, and libsystemd by default

v5.0 ships keventd and the D-Bus support enabled out of the box, but the
remaining bundled pieces stayed opt-in, and the help text for two of them
already claimed otherwise.

Default all three to yes; --without-sulogin, --without-watchdog, and
--without-libsystemd opt out.  The distcheck and CI configure lines drop
the flags they no longer need, so CI exercises the defaults.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/coverity.yml

@@ -66,8 +66,7 @@ jobs:
       - name: Configure
         run: |
           ./autogen.sh
-          ./configure --prefix= --enable-x11-common-plugin --with-watchdog \
-                      --with-keventd --with-libsystemd
+          ./configure --prefix= --enable-x11-common-plugin
       - name: Build
         run: |
           export PATH=`pwd`/coverity/bin:$PATH