Consolidate methods for translating a Perl path to a Windows path.

Consolidate methods for translating a Perl path to a Windows path.

This fixes some TAP suites when using msys Perl and a builddir located
in an msys mount point other than "/".  For example, builddir=/c/pg
exhibited the problem, since /c/pg falls in mount point "/c".
Back-patch to 9.6, where tests first started to perform such
translations.  In back branches, offer both new and old APIs.

Reviewed by Andrew Dunstan.

Discussion: https://postgr.es/m/20190610045838.GA238501@rfd.leadboat.com