Add support for transparent compilation

This means that software being ported should not need to be modified in
the usual case, as the libbsd headers will take over the standard
namespace and fill the missing gaps, and include the system headers.

To use this the new libbsd-transparent.pc file can be used through
pkg-config, which should end up doing the right thing.

include/bsd/err.h

@@ -29,9 +29,15 @@
 #define LIBBSD_ERR_H
 
 #include <sys/cdefs.h>
-#include <err.h>
+
 #include <stdarg.h>
 
+#ifdef LIBBSD_TRANSPARENT
+#include_next <err.h>
+#else
+#include <err.h>
+#endif
+
 __BEGIN_DECLS
 extern void warnc (int code, const char *format, ...);
 extern void vwarnc (int code, const char *format, va_list ap);