Allow disabling of subordinate IDs.

* configure.in: Add configure options --enable-subordinate-ids /
	--disable-subordinate-ids. Enabled by default.
	* lib/prototypes.h: Include <config.h> before using its macros.
	* lib/commonio.h, lib/commonio.c: Define commonio_append only when
	ENABLE_SUBIDS is defined.
	* lib/prototypes.h, libmisc/find_new_sub_gids.c,
	libmisc/find_new_sub_uids.c: Likewise.
	* lib/subordinateio.h, lib/subordinateio.c: Likewise.
	* libmisc/user_busy.c: Only check if subordinate IDs are in use if
	ENABLE_SUBIDS is defined.
	* src/Makefile.am: Create newgidmap and newuidmap only if
	ENABLE_SUBIDS is defined.
	* src/newusers.c: Check for ENABLE_SUBIDS to enable support for
	subordinate IDs.
	* src/useradd.c: Likewise.
	* src/userdel.c: Likewise.
	* src/usermod.c: Likewise.
	* man/Makefile.am: Install man1/newgidmap.1, man1/newuidmap.1,
	man5/subgid.5, and man5/subuid.5 only if ENABLE_SUBIDS is defined.
	* man/fr/Makefile.am: Install man1/newgidmap.1, man1/newuidmap.1,
	man5/subgid.5, and man5/subuid.5 (not translated yet).
	* man/generate_mans.mak: Add xsltproc conditionals
	subids/no_subids.
	* man/login.defs.d/SUB_GID_COUNT.xml: Add dependency on subids
	condition.
	* man/login.defs.d/SUB_UID_COUNT.xml: Likewise.
	* man/usermod.8.xml: Document options for subordinate IDs and
	reference subgid(5) / subuid(5) depending on the subids condition.

man/usermod.8.xml

@@ -389,7 +389,7 @@
 	  </para>
 	</listitem>
       </varlistentry>
-      <varlistentry>
+      <varlistentry condition="subids">
 	<term>
 	  <option>-v</option>, <option>--add-sub-uids</option>
 	  <replaceable>FIRST</replaceable>-<replaceable>LAST</replaceable>
@@ -408,7 +408,7 @@
 	  </para>
 	</listitem>
       </varlistentry>
-      <varlistentry>
+      <varlistentry condition="subids">
 	<term>
 	  <option>-V</option>, <option>--del-sub-uids</option>
 	  <replaceable>FIRST</replaceable>-<replaceable>LAST</replaceable>
@@ -429,7 +429,7 @@
 	  </para>
 	</listitem>
       </varlistentry>
-      <varlistentry>
+      <varlistentry condition="subids">
 	<term>
 	  <option>-w</option>, <option>--add-sub-gids</option>
 	  <replaceable>FIRST</replaceable>-<replaceable>LAST</replaceable>
@@ -448,7 +448,7 @@
 	  </para>
 	</listitem>
       </varlistentry>
-      <varlistentry>
+      <varlistentry condition="subids">
 	<term>
 	  <option>-W</option>, <option>--del-sub-gids</option>
 	  <replaceable>FIRST</replaceable>-<replaceable>LAST</replaceable>
@@ -588,12 +588,14 @@
       <citerefentry>
 	<refentrytitle>login.defs</refentrytitle><manvolnum>5</manvolnum>
       </citerefentry>,
-      <citerefentry>
-	<refentrytitle>subgid</refentrytitle><manvolnum>5</manvolnum>
-      </citerefentry>,
-      <citerefentry>
-	<refentrytitle>subuid</refentrytitle><manvolnum>5</manvolnum>
-      </citerefentry>,
+      <phrase condition="subids">
+	<citerefentry>
+	  <refentrytitle>subgid</refentrytitle><manvolnum>5</manvolnum>
+	</citerefentry>,
+	<citerefentry>
+	  <refentrytitle>subuid</refentrytitle><manvolnum>5</manvolnum>
+	</citerefentry>,
+      </phrase>
       <citerefentry>
 	<refentrytitle>useradd</refentrytitle><manvolnum>8</manvolnum>
       </citerefentry>,