Fix typos and inconsistencies for HEAD (take 6)

Hello hackers,

Please consider fixing the next batch of typos and inconsistencies in
the tree:
6.1. FADVISE_WILLNEED -> POSIX_FADV_WILLNEED
6.2. failOK -> missing_ok
6.3. failOnerror -> failOnSignal
6.4. fakebits -> remove (irrelevant since introduction in 945543d9)
6.5. FastPathGetLockEntry -> FastPathGetRelationLockEntry
6.6. FAST_PATH_HASH_BUCKETS -> FAST_PATH_STRONG_LOCK_HASH_PARTITIONS
6.7. FastPathTransferLocks -> FastPathTransferRelationLocks
6.8. GenericOptionFlags -> remove (unused since 090173a3)
6.9. fetch_data -> fetched_data
6.10. fildes -> fd
6.11. filedescriptors -> file descriptors
6.12. fillatt -> remove (orphaned since 8609d4ab)
6.13. finalfunction -> finalfn
6.14. flail -> fail
6.15. FlushBuffers -> FlushBuffer & rephrase a comment (incorrectly
updated in 6f5c38dc)
6.16. flush_context -> wb_context
6.17. followon -> follow-on
6.18. force_quotes -> remove (orphaned since e18d900d)
6.19. formatstring -> format-string
6.20. formarray, formfloat -> remove (orphaned since a237dd2b)
6.21. found_row_type -> found_whole_row
6.22. freeScanStack -> remove a comment (irrelevant since 2a636834)
6.23. free_segment_counter -> freed_segment_counter
6.24. FreeSpace Map -> FreeSpaceMap
6.25. user friendly-operand -> user-friendly operand
6.26. frozenids -> frozenxids
6.27. fsm_internal.h -> fsm_internals.h
6.28. fsm_size_to_avail_cat -> fsm_space_avail_to_cat
6.29. full_result -> full_results
6.30. FULL_SIZ -> remove (orphaned since 65b731bd)
6.31. funxtions -> functions
6.32. generate_nonunion_plan, generate_union_plan ->
generate_nonunion_paths, generate_union_paths
6.33. getaddinfo -> getaddrinfo
6.34. get_expr, get_indexdef, get_ruledef, get_viewdef, get_triggerdef,
get_userbyid -> pg_get_*
6.35. GetHashPageStatis -> GetHashPageStats
6.36. GetNumShmemAttachedBgworkers -> remove (orphaned since 6bc8ef0b)
6.37. get_one_range_partition_bound_string -> get_range_partbound_string
6.38. getPartitions -> remove a comment (irrelevant since 44c52881)
6.39. GetRecordedFreePage -> GetRecordedFreeSpace
6.40. get_special_varno -> resolve_special_varno
6.41. gig -> GB
6.42. GinITupIsCompressed -> GinItupIsCompressed
6.43. GinPostingListSegmentMaxSize-bytes -> GinPostingListSegmentMaxSize
bytes
6.44. gistfindCorrectParent -> gistFindCorrectParent
6.45. gistinserthere -> gistinserttuple
6.46. GISTstate -> giststate
6.47. GlobalSerializableXmin, SerializableGlobalXmin -> SxactGlobalXmin
6.48. Greenwish -> Greenwich
6.49. groupClauseVars -> groupClauseCommonVars

As a side note, while looking at dt_common.c (fixing 6.47), I've got a
feeling that the datetktbl is largely outdated and thus mostly unuseful
(e.g. USSR doesn't exist for almost 30 years).

Best regards,
Alexander