chaperone-evt: don't drop other chaperones
prop:evt: support chaperone when extracting field

Thanks to Sam for noticing this problem.
prop:evt, prop:{input,output}-port and evt chaperones: fix up scheduling

Although it doesn't seem to be possible currently, avoid the case
that a property-based access or chaperone is called in a scheduler
context.
pkg/lib: fix `pkg-directory->additional-installs`

Merge to v6.1 (with a70b3173b2)