ldpd: convert cli and get rid of the xml interface

ldpd: convert cli and get rid of the xml interface

The xml2cli.pl script was useful years ago when the vty code was very
rudimentary. This is not the case anymore, so convert all ldpd CLI
commands to use DEFUNs directly and get rid of the XML interface.

The benefits are:
* Consistency with the other daemons;
* One less build dependency (the LibXML perl module);
* Easier to add new commands.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>