update Homepage and Vcs URLs

upstream moved to https://github.com/gonicus/fts
do not fail to purge if the config file is not there
fix extracting the upstream version from a NMU changelog entry
fix debconf usage during purge
fix up /etc/fts/config handling
register and purge /etc/fts/config with ucf/ucfr

otherwise fts can't be purged and reinstalled properly
ship empty /var/lib/tftpboot directory