net-mgmt/rate: fix build on 15-CURRENT

• link with -lutil to grab ftime() on 15-CURRENT. This is harmless on older FreeBSD
• while we are at it, install binary using INSTALL_PROGRAM so it is stripped

Approved by: portmgr (build fix blanket)
MFH: 2024Q4

(cherry picked from commit 76b0011589e1ec4eb2ebf5544e47e4ea7d630a99)