scmutil: split platform-specific bits into their own modules
This parallels what's done for the util module, which imports either
mercurial.posix or mercurial.windows as 'platform' and then slurps the
appropriate functions into its own namespace.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html