dirname: move m4/dos.m4 functionality into lib/dosname.h

dirname: move m4/dos.m4 functionality into lib/dosname.h

m4/dos.m4 needs to go.  It laboriously invokes the C compiler, and
extracts symbols from it, puts them into config.h; but it's much
easier to use the symbols directly.  filename.h already does this,
but it disagrees with dos.m4 in some respects.  This patch
introduces a different include file dosname.h that packages up
dos.m4, and then later we can work on merging filename.h and
dosname.h.  Applications that need only the easy-to-configure
symbols should consider including dosname.h rather than dirname.h.
* NEWS: Mention incompatible changes.
* m4/dos.m4: Remove.
* lib/dosname.h, modules/dosname: New files.
* lib/dirname.h (ISSLASH, FILE_SYSTEM_PREFIX_LEN):
(FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE):
(IS_ABSOLUTE_FILE_NAME, IS_RELATIVE_FILE_NAME): Move to lib/dosname.h.
* lib/at-func.c, lib/at-func2.c, lib/openat.c, lib/savewd.c:
Include dosname.h, not dirname.h.
* lib/rmdir.c, lib/stat.c, lib/unlink.c, lib/unlinkat.c:
Include dosname.h, for definitions of symbols like ISSLASH
that used to be in config.h.
* m4/dirname.m4 (gl_DIRNAME_LGPL): Do not require gl_AC_DOS.
* m4/rmdir.m4 (gl_FUNC_RMDIR): Likewise.
* m4/stat.m4 (gl_FUNC_STAT): Likewise.
* m4/unlink.m4 (gl_FUNC_UNLINK): Likewise.
* modules/dirname-lgpl (Files): Omit m4/dos.m4.
* modules/rmdir (Files): Likewise.
* modules/stat (Files): Likewise.
* modules/unlink (Files): Likewise.
* modules/dirname-lgpl (Depends-on): Add dosname.
* modules/lstat (Depends-on): Likewise.
* modules/openat (Depends-on): Likewise.
* modules/rmdir (Depends-on): Likewise.
* modules/savewd (Depends-on): Likewise.
* modules/stat (Depends-on): Likewise.
* modules/unlink (Depends-on): Likewise.
* modules/openat (Depends-on): Remove dirname-lgpl.
* modules/savewd (Depends-on): Likewise.
* tests/test-dirname.c: Do not use removed symbols like
FILE_SYSTEM_BACKSLASH_IS_FILE_NAME_SEPARATOR.  Instead, use
the remaining symbols, e.g., ISSLASH ('\\').