patch 9.0.1330: handling new value of an option has a long "else if" chain

Problem:    Handling new value of an option has a long "else if" chain.
Solution:   Use a function pointer. (Yegappan Lakshmanan, closes #12015)
diff --git a/src/option.h b/src/option.h
index 56b489b..9cc540a 100644
--- a/src/option.h
+++ b/src/option.h
@@ -14,7 +14,7 @@
 #define _OPTION_H_
 
 //
-// Flags
+// Option Flags
 //
 #define P_BOOL		0x01	// the option is boolean
 #define P_NUM		0x02	// the option is numeric