sysutils/xxd: Make port track vim version, fix WWW

As suggested by adamw, since the port uses the vim distribution,
it can track the vim port PORTVERSION, and use its distfile. [1]

While here:

• Fix WWW, tracking vim one here too [1]
• Adjust COMMENT and pkg-plist a little

Reported by: adamw [1]
Approved by: Gianmarco Giovannelli <gmarco@giovannelli.it> (maintainer)