(Fexpand_file_name, Ffile_name_absolute_p): Doc fixes.

(Fexpand_file_name, Ffile_name_absolute_p): Doc fixes.
(Ffile_exists_p, Ffile_executable_p, Ffile_readable_p)
(Ffile_writable_p, Ffile_directory_p, Ffile_regular_p)
(Ffile_modes, Fset_file_modes, Ffile_newer_than_file_p)
(expand_and_dir_to_file): Rename abspath to absname.

(CORRECT_DIR_SEPS) [DOS_NT]: New macro.
(IS_DRIVE) [DOS_NT]: Added separate definitions for DOS and NT.
(Ffile_name_directory) [DOS_NT]: Simplify code to match change in
getdefdir in msdos.c.  Ignore embedded colons.  Correct dir seps.
(Ffile_name_nondirectory) [DOS_NT]: Ignore embedded colons.
Correct IS_ANY_SEP to IS_DIRECTORY_SEP.
(file_name_as_directory) [DOS_NT]: Correct dir seps.
Correct IS_ANY_SEP to IS_DIRECTORY_SEP.
(directory_file_name) [DOS_NT]: Correct dir seps.
(Fmake_temp_name) [DOS_NT: Correct dir seps.
(Fexpand_file_name) [mostly DOS_NT]: Remove relpath, tmp and
defdir variables; init drive to 0.
Correctly detect when default_directory is absolute.
Be strict when looking for MSDOS drive specifier; defer calling
getdefdir.  Ignore drive specifier if name now has UNC prefix.
Correctly recognise if name is not absolute when trying simple
method to expand; return original string if possible.
Skip dir sep after ~ or ~user.
Use getpwnam instead of HOME for ~user on NT.
Handle error return from getdefdir.
Correctly detect if newdir is absolute before using default_directory.
Handle case where newdir is not absolute - expand relative to
current working dir if necessary (instead of calling getdisk
later).  Only keep UNC prefix if nm starts with dir sep.
Replace kludgy handling of drive spec in newdir.  Correct dir seps.
(Fexpand_file_name) [!DOS_NT]: Fix incorrect expansion of
"/foo/../bar" -> "//bar".
(Fsubstitute_in_file_name) [DOS_NT]: Correct dir seps for NT as
well.  Merge equivalent #ifdef APOLLO and WINDOWSNT cases.  Ignore
embedded colons and be strict about drive specs.
(Fcopy_file) [DOS_NT]: Do dev/inode check on NT.
(Ffile_name_absolute_p) [DOS_NT]: Be strict about drive specs.
(check_executable) [DOS_NT]: Test st_mode on NT.
(Ffile_readable_p) [DOS_NT]: Use access instead of open on NT.
(Ffile_modes) [DOS_NT]: Don't embelish st_mode value on NT.
(Fread_file_name) [DOS_NT]: Correct dir seps in HOME.
(syms_of_fileio): Add Vdirectory_sep_char.