Clean up a bit for symlinked headers.

<errno.h> as well as <poll.h> are symlinked to their <sys/...>
counterparts, so there is no need to include both.

Also fix a comment that suggests that there i

Clean up a bit for symlinked headers.

<errno.h> as well as <poll.h> are symlinked to their <sys/...>
counterparts, so there is no need to include both.

Also fix a comment that suggests that there is a difference.

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usr.sbin: Remove duplicate includes.

installer: Move the installer from contrib/ to usr.sbin/.

Fix trailing whitespace while I'm doing it.