Make 410.pkg-audit more flexible

Get PKG_DBDIR from pkg -vv directly
Use /usr/sbin/pkg
Fixes: #517
Use /usr/sbin/pkg as it handles LOCALBASE properly
Substitute the value of PREFIX into periodic scripts
Revert "Substitute the value of PREFIX into periodic scripts"

This reverts commit 391e617d9f3ce1b1ad8789b1bdcfe06a5c69603a.
Merge branch 'master' of github.com:freebsd/pkg