Fix some unique identifiers/entities

Hello hackers,

I've stumbled upon a misspelled HAVE_ZLIB in a comment and decided to
check all the unique identifiers/entities in the source tree. Using the
balleyeing technique I've processed questionable A* and HAVE_* unicums
(for now). The patches for every one are attached.

1. AExprConst -> AexprConst (an inconsistent case)
2. AlterExtensionOwner_oid - remove (orphaned after 994c36e0)
3. AlterTableDropColumn -> ATExecDropColumn (renamed in 077db40f)
4. ApplySortComparatorFull -> ApplySortAbbrevFullComparator (an internal
inconsistency)
5. arracontjoinsel -> arraycontjoinsel (just a typo)
6. ArrayNItems -> ArrayGetNItems (an internal inconsistency)
7. ArrayRef & ArrayRefState -> SubscriptingRef & SubscriptingRefState
(renamed by 558d77f2)
8. AT_AddOids  - remove (orphaned after 578b2297)
10. AtPrepare_Inval - remove (orphaned after efc16ea52)
11. AttachIndexInfo -> IndexAttachInfo (an internal inconsistency)
12. AttributeOffsetGetAttributeNumber - > AttrOffsetGetAttrNumber (an
internal inconsistency)
13. AttInMetaData -> AttInMetadata (an inconsistent case)
14. AUTH_REQ_GSSAPI -> AUTH_REQ_GSS (an internal inconsistency)
15. authenticaion -> authentication (a typo)
16. HAVE__BUILTIN_CLZ -> HAVE__BUILTIN_CLZ (a typo)
17. HAVE_BUILTIN_CLZ -> HAVE__BUILTIN_CLZ (a typo)
18. HAVE_BUILTIN_CTZ -> HAVE__BUILTIN_CLZ (a typo)
18. HAVE_FCVT - remove (survived after ff4628f3)
19. HAVE_FINITE - remove (orphaned after cac2d912)
20. HAVE_RAND_OPENSSL - remove (orphaned after fe0a0b59)
21. HAVE_STRUCT_SOCKADDR_UN - remove (survived after ff4628f3)
22. HAVE_SYSCONF - remove (survived after ff4628f3)
23. HAVE_ZLIB -> HAVE_LIBZ (a typo)

I hope you will find it useful. If so, I can continue this work.

Best regards,
Alexander