ftp/vsftpd-ext: unbreak build on 9.x i386 and clean up port

• Force clang on FreeBSD < 10.0 to resolve 9.x i386 build issues
• Modernize USES with tar:tgz
• Rely on USERS/GROUPS; remove pkg-install/pkg deinstall scripts
• Remove direct chmod usage
• Regen patch to pet portlint

PR: 200791
Submitted by: Xu Jing <xjflyttp@gmail.com> (maintainer - original version)
Approved by: delphij (mentor)
MFH: 2015Q3