Move value_true to value.h

I noticed that value_true is declared in language.h and defined in
language.c.  However, as part of the value API, I think it would be
better in one of those files.  And, because it is very short, I
changed it to be an inline function in value.h.  I've also removed a
comment from the implementation, on the basis that it seems obsolete
-- if the change it suggests was needed, it probably would have been
done by now; and if it is needed in the future, odds are it would be
done differently anyway.

Finally, this patch also changes value_true and value_logical_not to
return a bool, and updates some uses.

gdb/cli/cli-script.c

@@ -19,7 +19,6 @@
 
 #include "defs.h"
 #include "value.h"
-#include "language.h"		/* For value_true */
 #include <ctype.h>
 
 #include "ui-out.h"
@@ -579,7 +578,7 @@ execute_control_command_1 (struct command_line *cmd, int from_tty)
 	/* Keep iterating so long as the expression is true.  */
 	while (loop == 1)
 	  {
-	    int cond_result;
+	    bool cond_result;
 
 	    QUIT;