add lxc.haltsignal for soft shutdown

- use this in the busybox template since busybox's init expects
  to receive SIGUSR1 to halt

- fix lxc.stopsignal to be output by write_config so lxcapi_clone()
  and lxcapi_save_config() will output it

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
configure.ac: add docbook-to-man to dbparsers

Debian and Ubuntu uses docbook2x-man, but some other distr like suse
uses docbook-to-man. I think all of them should work on LXC.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
allow multiple types of init scripts to be configured

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>