login_prompt: Simplify login_prompt API

The only user of login_prompt is the login tool. This implies that the
first argument is always the same.

It is much easier to verify printf's format string and its argument if
both are next to each other.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>

lib/prototypes.h

@@ -233,7 +233,7 @@ extern void dolastlog (
 extern int login_access (const char *user, const char *from);
 
 /* loginprompt.c */
-extern void login_prompt (const char *, char *, int);
+extern void login_prompt (char *, int);
 
 /* mail.c */
 extern void mailcheck (void);