Minor cleanups in slot related code.

Andres Freund <andres@anarazel.de>

Commit: 3b425b7c02ccdabe3fb3a538d9174273a15f1e2e
Author: Andres Freund <andres@anarazel.de>
Date: 2015-08-11T10:32:48Z
Releases: 9.6.0
Minor cleanups in slot related code.

Fix a bunch of typos, and remove two superflous includes.

Author: Gurjeet Singh
Discussion: CABwTF4Wh_dBCzTU=49pFXR6coR4NW1ynb+vBqT+Po=7fuq5iCw@mail.gmail.com
Backpatch: 9.4

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