Implement RENAME-FILE

Implement RENAME-FILE

This introduces a primitive (RENAME-FILE) which takes C-strings as
arguments.  The conversion from Forth-strings to C-strings is done in
Forth code as it doesn't seem to be any easier to do it in C.

* csrc/pf_io.h (sdRenameFile): New.  It has the same semantics as
rename(2) from <stdio.h>

* csrc/pf_io.c (sdRenameFile): New stub.

* csrc/pf_guts.h (cforth_primitive_ids): ID_FILE_RENAME added,
ID_RESERVED12 removed

* csrc/pf_inner.c: Add case for ID_FILE_RENAME.

* csrc/pfcompil.c (pfBuildDictionary): Create entry for (RENAME-FILE).

* fth/file.fth (RENAME-FILE): New.
(THROW_RENAME_FILE): New constant.

* fth/t_file.fth: Remove stub and uncomment some tests.