Used shell built-in command -v to determine if dig is installed and if
it is, use it as default for getting the ip-address. Also made the use
of brackets consistent and updated the the way the data variable is
created before it is sent with curl to make it more readable in the
code.
Permits to run the script on simpler/lighter Linux distributions,
e.g. OpenWRT, where the grep utility available does not permit to use
Perl extended regexp (the `grep -P` option)
Instead of creating a log file. This change leverages the syslog. This allows us to not have to manually rotate logs, or worry about the log file filling up the file system. This also give us timestamps of the logging without having to create the timestamps ourselves. Messages are logged to /var/log/messages by default.