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