Rename all source files to 8.3 format for DOS

[philburk]

Some files do not follow the 8.3 DOS filename convention.
This is still causing problems for some DOS builds!

Originally posted by @philburk in #201 (comment)

[philburk]

Here are the files to be renamed:

src/csrc/posix/pf_io_posix.c src/csrc/posix/ioposix.c
src/csrc/stdio/pf_fileio_stdio.c src/csrc/stdio/fiostdio.c
src/csrc/stdio/pf_io_stdio.c src/csrc/stdio/iostdio.c
src/csrc/win32/pf_io_win32.c src/csrc/win32/iowin32.c
src/csrc/win32csl/pf_io_win32_console.c src/csrc/win32csl/iowin32c.c
src/fth/save-input.fth src/fth/savinput.fth
src/fth/structure.fth src/fth/structur.fth
src/fth/t_include.fth src/fth/tinclude.fth
src/fth/t_load_defer.fth src/fth/tlddefer.fth
src/fth/t_load_pairs.fth src/fth/tldpairs.fth
src/fth/t_load_semi.fth src/fth/tldsemi.fth
src/fth/t_load_undef.fth src/fth/tldundef.fth
src/fth/t_required_helper1.fth src/fth/treqhlp1.fth
src/fth/t_required_helper2.fth src/fth/treqhlp2.fth
src/fth/t_strings.fth src/fth/tstrings.fth
src/fth/utils/dump_struct.fth src/fth/utils/dumpstru.fth
src/fth/utils/load_file.fth src/fth/utils/loadfile.fth
src/fth/utils/make_all256.fth src/fth/utils/make_all.fth