Unify gdb putc functions

Now that filtered and unfiltered output can be treated identically, we
can unify the putc family of functions.  This is done under the name
"gdb_putc".  Most of this patch was written by script.

gdb/linux-fork.c

@@ -597,7 +597,7 @@ info_checkpoints_command (const char *arg, int from_tty)
 	    printf_filtered (", <%s>", msym.minsym->linkage_name ());
 	}
 
-      putchar_filtered ('\n');
+      gdb_putc ('\n');
     }
   if (printed == NULL)
     {