Deprecate ASSUAN_SYSTEM_NPTH.

* src/assuan.h.in: Introduce new configuration with
ASSUAN_REALLY_REQUIRE_OLD_WAY_OF_SYSTEM_NPTH.

Existing use cases just works with ASSUAN_SYSTEM_NPTH=NULL.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>