MFC r314321:
dc(1): Merge minor changes from OpenBSD.

Prefer setvbuf() to setlinebuf() for portability.
Some style(9) and redundant tests for NULL.

These are only meant to ease up merging newer changes.

Obtained from: OpenBSD

MFC r314316:
dc(1): Catch up with OpenBSD tag.

OpenBSD rev 1.12 corresponds to our SVN r275162. Update the tag to make
easier future updates. No functional change.

MFC r275162:
Init array field in the proper place.

Obtained from: OpenBSD

MFC r275162:
Init array field in the proper place.

Obtained from: OpenBSD