clamav: add page (#4423)

pages/linux/clamav.md

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+# clamav
+
+> Open-source anti-virus program.
+> Designed especially for e-mail scanning on mail gateways, but can be used in other contexts.
+> More information: <https://www.clamav.net>.
+
+- Update virus definitions:
+
+`freshclam`
+
+- Scan a file for viruses:
+
+`clamscan {{path/to/file}}`
+
+- Scan directories recursively and print out infected files:
+
+`clamscan --recursive --infected {{path/to/directory}}`
+
+- Scan directories recursively and move them into quarantine:
+
+`clamscan --recursive --move={{directory}}`