installkernel: Add support for hooks under /usr/share/kernel/postinst.d

Ben Hutchingsrequested to merge
benh/debianutils:installkernel-hooks-in-usr into master

Unlike the other implementations, installkernel can rely on the version of run-parts installed, so this unconditionally passes both directory names to run-parts.

Closes: #1107113

Edited by Ben Hutchings

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