Fix and improve filemon(4) example: - remove return statements from void function [1] - include missing header - use O_CLOEXEC instead of separate fcntl() calls
Fix and improve filemon(4) example:
- remove return statements from void function [1]
- include missing header
- use O_CLOEXEC instead of separate fcntl() calls

PR:		docs/179459 [1]
MFC after:	1 week

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