Make doprnt and related functions ANSI C compliant, with prototypes.

Make doprnt and related functions ANSI C compliant, with prototypes.

* doprnt.c (doprnt): Take a va_list argument instead of count and
pointer.
* eval.c (error): Change to a standard-C variadic function.
* xdisp.c (vmessage): Renamed from message, made static, and
changed to take a va_list argument.
(message): New variadic wrapper.
(message_nolog): Now a variadic function, calling vmessage.
* lisp.h: Include stdarg.h for va_list.
(doprnt, error, message, message_nolog): Decls updated.