Before the addition of the okmask, we could only selectively turn off
optimizations for the built-in housekeeping subs (vcl_init {} /
vcl_fini {}), not for any custom subs called from there.

The previous commit changed this and turned off optimizations for any
custom subs called from there.

We pull this tighter again by only turning off optimizations for subs
which can _only_ be called from housekeeping.

Ref f1c47e48
Ref #3230