security/openvpn: Needs addition to SHEBANG_FILES and some cleanup

• Remove unnecessary GNU_CONFIGURE_MANPREFIX.
• Improve post-install-EXAMPLES-on.

PR: 296890
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>
MFH: 2026Q3

(cherry picked from commit 83283c284c4a9dd90ba4ce23a50d1cfb1005dbdf)