stat: Drop unused code and conditionals

We haven't pulled from upstream in over 15 years, and the codebases
have diverged so far it is unlikely that we ever will.

• Drop NetBSD and OpenBSD version control information.

• Drop support for building on non-BSD / non-POSIX platforms.

• Fix a few minor style issues.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D56771