fix building docs

fix building docs

Commit 69fe23ff added checking for the older docbook2man back into
configure, but this breaks building the docs on at least Oracle Linux and
Fedora when docbook2X is not installed as docbook2man will be found but the
docs don't actually build with that tool.

This change makes it so the docs can be built with either the older
docbook2man or the newer 2X tools by using configure to set the dtd
string to an appropriate value depending on use of docbook2man or
db2x_docbook2man.

Also fixed a small error in lxc-destroy.sgml.in that was noticed
by the old tools.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>