CLAMSCAN_OPTIONS: make possible to customize clamscan parameters

[kerphi]

Change the default parameters to: --recursive=yes --allmatch=yes --remove=no --suppress-ok-results

Then add a parameter CLAMSCAN_OPTIONS making possible to customize clamscan command.

As a result we should have:

clamscan --recursive=yes --allmatch=yes --remove=no --suppress-ok-results  /folder-to-scan/