opensnoop: add page (#1576)

pages/osx/opensnoop.md

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+# opensnoop
+
+> Tool that tracks file opens on your system.
+
+- Print all file opens as they occur:
+
+`sudo opensnoop`
+
+- Track all file opens by a process by name:
+
+`sudo opensnoop -n {{process_name}}`
+
+- Track all file opens by a process by PID:
+
+`sudo opensnoop -p {{PID}}`
+
+- Track which processes open a specified file:
+
+`sudo opensnoop -f {{path/to/file}}`