net/akhq: Clean up port

In Makefile:

• Better COMMENT
• Use ${ETCDIR} when applicable
• Update PLIST_FILES
• Removed post-install part

In service file:

• Use %WWWOWN%%

In sample configuration file, bind to 127.0.0.1 instead of 0.0.0.0

Sponsored by: Klara, Inc.
Approved by: 0mp (mentor)
Differential Revision: https://reviews.freebsd.org/D48886