Fix DOS build; don't override convert-standard-filename.

 config.bat: Remove support for DJGPP v1.x.
 src/Makefile.in: Fix MSDOS-related comments.
 msdos/sed3v2.inp (INSTALLABLES): Edit out extra ${EXEEXT} after "emacsclient".
 (emacsserver, timer, wakeup): Remove edit-out commands.
 msdos/sed1v2.inp (MSDOS_OBJ): Add w16select.o.
 (TERMCAP_OBJ): Add termcap.o.
 lisp/files.el (convert-standard-filename): Call
 w32-convert-standard-filename and dos-convert-standard-filename on
 the corresponding systems.
 lisp/w32-fns.el (w32-convert-standard-filename): Rename from
 convert-standard-filename.  Doc fix.
 lisp/dos-fns.el (dos-convert-standard-filename): Doc fix.
 (convert-standard-filename): Don't defalias.
 (register-name-alist, make-register, register-value)
 (set-register-value, intdos): Obsolete aliases for the
 corresponding dos-* functions and variables.
 (dos-intdos): Add a doc string.