common: Use gpgrt functions for mkdir and chdir if available

* common/sysutils.c (gnupg_mkdir): Divert to gpgrt_mkdir.
(gnupg_chdir): Divert to gpgrt_chdir

To avoid bumping up the build dependency on libgpg-error 1.28 we use
the gpgrt version only if at least this libgpg-error version was used
at build time. This won't fix any bugs though and it is in general
advisable to use the latest libgpg-error. There are actually a couple
of very useful bug fixes for Windows in the upcoming libgpg-error 1.39
but on Unix you can live without them.

• Signed-off-by: Werner Koch <wk@gnupg.org>