Don't override standard definition of convert-standard-filename.

Don't override standard definition of convert-standard-filename.
Add obsolete aliases for dos-fns.el functions.

 files.el (convert-standard-filename): Call
 w32-convert-standard-filename and dos-convert-standard-filename on
 the corresponding systems.
 w32-fns.el (w32-convert-standard-filename): Rename from
 convert-standard-filename.  Doc fix.
 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.