Resolve id conflicts in See Also sections in valgrind and vgdb manpages.

Because the manpages are processed together they cannot contain the
same ids. Both the valgrind and vgdb manpage reference the vgdb main
manual URL. That conflicts even though the valgrind.1 and vgdb.1 manual
page are separate. Prefixing the vgdb ids (with "vgdb-") works around
the conflict. It still works fine, since in vgdb the references are only
directly used in the "See Also" refsect. The labels and urls still come
out as intended.

With this fix make valid validates both the manual index.xml and
manpages-index.xml without errors.

Author: Mark Wielaard

coregrind/docs/vgdb-manpage.xml

@@ -57,13 +57,18 @@ intermediary between Valgrind and GDB or a shell.  It has two usage modes:
 valgrind(1),
 <filename>&vg-docs-path;</filename> or
 <filename>&vg-docs-url;</filename>,
-<ulink id="&vg-gdbserver-id;"
+<!-- Note that the ids used have vgdb- prefixed.  This is because the same
+     ids are used in the valgrind.1 man page.  That conflict even though
+     the valgrind.1 and vgdb.1 manual page are generated separately.  It
+     still works fine, since in vgdb the references are only direct in this
+     "See Also" refsect. The labels and urls still come out as intended.  -->
+<ulink id="vgdb-&vg-gdbserver-id;"
        xreflabel="&vg-gdbserver-label;"
        url="&vg-gdbserver-url;">&vg-gdbserver-label;</ulink>
-<ulink id="&vg-vgdb-id;"
+<ulink id="vgdb-&vg-vgdb-id;"
        xreflabel="&vg-vgdb-label;"
        url="&vg-vgdb-url;">&vg-vgdb-label;</ulink>,
-<ulink id="&vg-monitor-id;"
+<ulink id="vgdb-&vg-monitor-id;"
        xreflabel="&vg-monitor-label;"
        url="&vg-monitor-url;">&vg-monitor-label;</ulink>.
 </para>