Move cord_pos.h to public headers folder (since included from cord.h)

Move cord_pos.h to public headers folder (since included from cord.h)

* include/private/cord_pos.h: Rename to include/cord_pos.h.
* BCC_MAKEFILE (de_win.obj): Rename "private/cord_pos.h" entry to
"cord_pos.h".
* Makefile.direct (CORD_SRCS, CORD_INCLUDE_FILES): Likewise.
* Makefile.dj (CORD_SRCS, CORD_INCLUDE_FILES): Likewise.
* NT_MAKEFILE (de_win.obj): Likewise.
* NT_STATIC_THREADS_MAKEFILE (de_win.obj): Likewise.
* NT_X64_STATIC_THREADS_MAKEFILE (de_win.obj): Likewise.
* NT_X64_THREADS_MAKEFILE (de_win.obj): Likewise.
* OS2_MAKEFILE (cordbscs.obj, cordxtra.obj, cordprnt.obj,
cordtest.exe): Likewise.
* gc.mak (DEP_CPP_DE_WI, DEP_CPP_DE_C2e, DEP_CPP_CORDX,
DEP_CPP_CORDB): Likewise.
* include/include.am (dist_noinst_HEADERS): Likewise.
* include/cord.h: Include "private/cord_pos.h" instead of "cord_pos.h".
* include/cord_pos.h (CORD_H): Test (skip the file if undefined).
* include/cord_pos.h (CORD_POSITION_H): Define.