ports-mgmt/pkg: update to 1.20.9

Changes:

• fix a race in vuln.xml udpate trigger
• undocument ftp support
• curl: add backward compatibility with libfetch on HTTP_USER_AGENT
• new Keyword: @override_prefix (pkgbase)
• only generate .pkgsave files when really needed

(cherry picked from commit a8de99feb6f55787ef1ac2c75cb65575f67a71e4)