Log behaviour if no log file path is provided

Robustness of the log function inproved.
If no log file path is provided, the function echos the log data to the
standard output
script signature changed and better robustness

the mandatorry parameters (fs and threshold) are now optional
If the fs is not defined, all ZFS fs are monitored
If the threshold is not defined 80% is assumed
The script now checks the validity of the arguments