[MSDOS]: #include "msdos.h" and <sys/param.h> needed for

[MSDOS]: #include "msdos.h" and <sys/param.h> needed for
the following changes.
(Ffile_name_directory, Fexpand_file_name) [FILE_SYSTEM_CASE]: Apply
case conversion if defined.
(Ffile_name_directory, Ffile_name_nondirectory, file_name_as_directory,
directory_file_name, Fexpand_file_name, Fsubstitute_in_file_name,
expand_and_dir_to_file) [MSDOS]: Drive letter support.
(Fexpand_file_name) [MSDOS]: Support for multiple default directories.
(Ffile_writeable_p) [MSDOS]: Don't call access with file name ending in slash.
(Finsert_file_contents) [MSDOS]: Determine file type by name (call
find-buffer-file-type) and change CR+LF to LF if it is a text file.
(Fwrite_region) [MSDOS]: Use text/binary mode as specified by buffer_file_type.
(syms_of_fileio) [MSDOS]: Set Qfind_buffer_file_type.
(Fsubstitute_in_file_name) [MSDOS]: Ignore case in environtment variable.