allow placeholder=P jobs to be allowed=A as well

[npf]

use case:

P=x / A=x mean -t placeholder=x -t / allowed=x

job1 P=x is scheduled
add job2 P=y A=x → can overlap job1
job3 A=x cannot overlap job2 but job1
job4 A=y cannot overlap job1 but job2

but then if you drop job1, job4 can overlap the part of job1 that was overlapped by job2

⇒ this is a way to transition from one placeholder (e.g. maintenance) to another (e.g. test) avoiding other jobs (not allowed) to be scheduled in between.