Get rid of the dependency to an external tail command

Get rid of the dependency to an external tail command

The real time report relied on an external "tail" command to get the last
lines of the input log file. That dependency has been removed.

Moreover, if several log files were passed, the real time report would
always read the first one. Now, sarg chooses the most recently modified log
file.

The input log file can be in any supported format. It doesn't have to be
a standard squid log format.