systemd: Flip -D userdb to true

GDM 49 will use it.

There seems no additional dependencies here so we need to edit LFS
anyway (at least we'll need to remove the incorrect "with dependencies
that do not fit in LFS").  And the increased SBU is negligiable, and
enabling it in LFS (instead of BLFS) has an advantage that "systemctl
preset-all" will automatically enable it.  Thus adding it in LFS
instead of only BLFS.

chapter08/systemd.xml

@@ -64,7 +64,7 @@ meson setup ..                \
       -D sysusers=false       \
       -D rpmmacrosdir=no      \
       -D homed=disabled       \
-      -D userdb=false         \
+      -D userdb=true          \
       -D man=disabled         \
       -D mode=release         \
       -D pamconfdir=no        \
@@ -141,14 +141,21 @@ meson setup ..                \
       </varlistentry>
 
       <varlistentry>
-        <term><parameter>-D homed=disabled</parameter> and
-        <parameter>-D userdb=false</parameter></term>
+        <term><parameter>-D homed=disabled</parameter></term>
         <listitem>
-          <para>Remove two daemons with dependencies that do not fit
+          <para>Remove a daemon with dependencies that do not fit
           within the scope of LFS.</para>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><parameter>-D userdb=true</parameter></term>
+        <listitem>
+          <para>Enable the <command>systemd-userdbd</command> daemon that
+          GDM in BLFS will use.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><parameter>-D man=disabled</parameter></term>
         <listitem>
@@ -292,7 +299,8 @@ ninja test</userinput></screen>
         systemd-socket-activate, systemd-stdio-bridge, systemd-sysext,
         systemd-tmpfiles, systemd-tty-ask-password-agent,
         systemd-umount (symlink to systemd-mount),
-        telinit (symlink to systemctl), timedatectl, and udevadm</seg>
+        telinit (symlink to systemctl), timedatectl, udevadm, and
+        userdbctl</seg>
         <seg>libnss_myhostname.so.2, libnss_mymachines.so.2,
         libnss_resolve.so.2, libnss_systemd.so.2,
         libsystemd.so, libsystemd-shared-&systemd-version;.so (in /usr/lib/systemd),
@@ -866,6 +874,16 @@ ninja test</userinput></screen>
         </listitem>
       </varlistentry>
 
+      <varlistentry id="userdbctl">
+        <term><command>userdbctl</command></term>
+        <listitem>
+          <para>Inspects users, groups and group memberships</para>
+          <indexterm zone="ch-system-systemd userdbctl">
+            <primary sortas="b-userdbctl">userdbctl</primary>
+          </indexterm>
+        </listitem>
+      </varlistentry>
+
       <varlistentry id="libsystemd">
         <term><filename class="libraryfile">libsystemd</filename></term>
         <listitem>