Unset logfilesize rather than let it be zero

1) When saving Status: System logs: Settings, if the Log File Size field is left blank, it gets cast to (int) and ends pu as zero "0" in config.xml. That breaks the creation of clog files in /etc/rc during boot. /etc/rc expects logfilesize to be unset, blank or a valid integer >= 5120.
So, if the field is empty, then make sure that it is unset in config.xml
2) For data items that are cast to (int) whenputting into the config (nentries and logfilesize), copy the resulting value back to the $pconfig array. When the user enters 123.45 the in field, is correctly truncated to (int) 123 in config.xml. By copying that back to $pconfig, the "123" also appears correctly on the GUI page after the save.
3) Mention the units of measure of Log File Size (Bytes)
4) Mention that the user should press Save first when changing values, then they can use "Reset Log Files" to set the log file sizes to the just-saved value.
5) Display a message when the log files have been reset - it's a bit more user-friendly IMHO.