Unset all MH environment variables that aren't set below. (cherry picked from commit 16dba0d07e94832ef9a077e6524714d0e433a111)
Unset all MH environment variables that aren't set below.
(cherry picked from commit 16dba0d07e94832ef9a077e6524714d0e433a111)

dscho/nmh,mcr/nmh,dscho/nmh,mcr/nmh,dscho/nmh,dscho/nmh