ldpd: do a full configuration reload upon receiving a SIGHUP

ldpd: do a full configuration reload upon receiving a SIGHUP

The rationale here is to do something very similar to what the original
OpenBSD's ldpd(8) does to reload configuration files:
* create a new empty base configuration (vty_conf);
* parse the configuration file and store the result in vty_conf;
* merge the new configuration into the current configuration.

To do this, introduce a new global variable called "sighup" so that ldpd
doesn't attempt to merge vty_conf into ldpd_conf for each command in
the configuration file, which would be very disruptive. vty_conf should
be merged only after the whole configuration file is parsed in order to
avoid taking actions like shutting down neighbors when not necessary.

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