Revise and simplify Module::Build::Platform::VMS::_detildefy.

We really just need to make sure we don't take the home directory
from both $ENV{HOME} and the incoming tilde spec.  We were making
~foo/ into /Users/foo/../foo, which doesn't make a whole lot of
sense.
Get the last tilde.t test passing on VMS.

The only we really need now is because getpwuid doesn't take into
account $ENV{HOME} having been overridden and doesn't return the
home directory in Unix format.