Convert stripansi() from unsigned char to char

This silences a few compiler warnings.

This also cleans the function up by removing some always-true checks.
I'm not convinced that this function makes a whole lot of sense, though.
Will look into it further...

include/misc.h

@@ -75,7 +75,7 @@ extern LastMsg last_sent_ctcp[2];
 	void	BX_userage		(char *, char *);
 	void	stats_k_grep_end (void);
 	char	*stripansicodes (const char *);
-	char	*stripansi	(unsigned char *);
+	char	*stripansi	(const char *);
 	NickTab	*BX_gettabkey (int, int, char *);
 	void	BX_addtabkey (char *, char *, int);
 	void	clear_array (NickTab **, char *);