security/fswatch: Take maintainership & clean up

• Take maintainership
• Remove now empty files/
• Do not use SUB_FILES for pkg-message as there is nothing to substitute
• Set LICENSE and LICENSE_FILE
• Sort variables
• Do not set WRKSRC as it is not necessary
• Reword pkg-message
• Use @sample for a sample configuration file and bump port revision

Reviewed by: krion
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D17669