Make perl_setup.com infer perl_root from its own location.

The perl_setup.com command procedure initializes the root logical
name, the shareable image logical name pointing to the main Perl
dynamic library, and either command symbols or command table
entries that reference those logical names.  It's always by
default hard-coded the prefix chosen at configuration time as the
basis for where Perl should be located.

The original rationale for that was that it gets run right before
installperl and sets things up correctly for installation.  But
that optimizes for a single use case that is easily handled by
specifying the correct install location as a parameter and
pessimizes for every other use case, notably relocating an
installed Perl directory tree.

So make it infer the correct root from its own location and
specify the install location only at installation time.  Among
other benefits, this allows the install location to be chosen
at installation time by overriding the default prefix like so:

  MMK/MACRO=(PREFIX="DSA0:[homedir.MyPerl2.]") install