Update the VERSION file to be 1.8.2a1.

NOTE: The v1.8.1 release is going to come from a separate branch
because:

a) it is a spot-fix release; it will only include one critical bug fix
   as compared to the v1.8[.0] release
b) this /branches/v1.8 branch has moved considerably beyond where the
   v1.8[.0] release was cut
c) this /branches/v1.8 branch is showing several MPI-layer regressions
   compared to the v1.8[.0] release

Hence, this branch contains the v1.8[.0] release, and is intended to
contain all v1.8.x releases >= v1.8.2.  The v1.8.1 release will come
from /branches/v1.8.1.

See commit message on r31474 (in /branches/v1.8.1) for more
information.
Refs #4536: Move r31421 to v1.8 branch (Ensure that the session directory)

This orterun(1) update really should have been part of #4536.
Doc fix; needs no review.

---svn-pre-commit-ignore-below---

r31477 [[BR]]
orterun.1in: Document --allow-run-as-root option

Add some verbiage about how mpirun now defaults to disallowing running
as root, but you can use the --allow-run-as-root option to override
this default behavior.

Refs #4536