Add proper const keywords to sysctl(3) parameters.

Add proper const keywords to sysctl(3) parameters.

  Import some sysctl(3) patches from FreeBSD.

  - r204170 change the sysctl(3) and sysctlbyname(3) interfaces so they
    are in sync with FreeBSD and NetBSD.
  - r240176 Avoid a potential segfault in libc when the `name' argument
    is invalid.
  - r204172 & r244153 are cosmetics changes.

  Here follow the commit message from r204170 and r240176:

  - r204170: Add proper const keywords to sysctl(3) parameters.

  The `name' and `newp' arguments can be marked const, because the
  buffers
  they refer to are never changed. While there, perform some other
  cleanups:

  * Implement sysctlbyname() using sysctlnametomib() to prevent
    duplication of an undocumented kernel interface.
  * Fix some whitespace nits.

  - r240176: Avoid segfault if name is invalid.

  Basically, only check for CTL_USER if the sysctl fails with ENOENT.