Security Fix: correct insecure handling of .serc

The new behaviour is to ignore .serc files that aren't owned by the euid
of the se process and .serc files that are writable by other users.

Thank you to Matthias Kilian of the OpenBSD project for alerting me to
this issue.

Import se-3.0

Import se-2.2

Import se-2.1

Import se-2.0

Import se-1.0