Fix typos for v12 - Mailing list pgsql-hackers

Hello hackers,

I've done another round of cross-checking the master branch for new
unique identifiers/words. As my previous attempt to fix things was not
noticed, now I'm focusing on distinct typos.
1. authenticaion (user-visible string)
2. becuase
3. checkinunique
4. cheep
5. comparion (user-visible)
6. comparision
7. compatiblity
8. continuescanthat
9. current_locked_pid (user-visible)
10. essentally
11. exptected
12. funcation
13. guarantess
14. HEAP_HASOID
15. Interfact
16. minimalslotslot (similar to heapslot)
17. modifcations
18. multiplcation
19. optimised
20. pased
21. perfer
22. relvant
23. represnting
24. ski p
25. unexcpected (user-visible string)

I still hope such fixes are useful and will be accepted.

Best regards,
Alexander