loader: userboot: provide a getsecs() implementation

We don't need it for userboot, but it avoids issues with BIND_NOW, so
just provide it. time(3) isn't defined but ends up being provided by
libc linked into the host process, which is generally fine.

PR: 262920
Reviewed by: imp, jhb
MFC after: 3 days
Diferential Revision: https://reviews.freebsd.org/D34758