Lower privilege level of programs calling regression_main

On Windows this mean that the regression tests can now safely and
successfully run as Administrator, which is useful in situations like
Appveyor. Elsewhere it's a no-op.

Backpatch to 9.5 - this is harder in earlier branches and not worth the
trouble.

Discussion: https://postgr.es/m/650b0c29-9578-8571-b1d2-550d7f89f307@2ndQuadrant.com