Read data.cdb on signal SIGUSR1

Commit 'f7ac43f71f22' changed 'respond' routine to read 'data.cdb'
file ones before serving any requests. This patch adds support to
enable reloading of the 'data.cdb' upon receiving signal SIGUSR1.
This will help tinydns(8) server to serve up-to-date information
without having to restart.

Updated manual page for tinydns(8) to document this change and usage
of signal SIGUSR1.