ACCEPTED BUT PARSED INCORRECTLY

When we are setting the value in a system variable, 
We can set it like 

set sys_var="Iden1.Iden2";		//1
set sys_var='Iden1.Iden2';		//2
set sys_var=Iden1.Iden2;		//3
set sys_var=.ident1.ident2; 		//4
set sys_var=`Iden1.Iden2`;		//5


While parsing, for case 1(when ANSI_QUOTES is enable) and 2,
we will take as string literal(we will make item of type Item_string).
for case 3 & 4, taken as Item_field, where Iden1 is a table name and
iden2 is a field name.
for case 5, again Item_field type, where iden1.iden2 is taken as
field name.


Now in case 1, when we are assigning some value to system variable
(which can take string or enumerate type data), we are setting only 
field part.
This means only iden2 value will be set for system variable. This 
result in wrong result.

Solution:

(for string type) We need to Document that we are not allowed to set 
system variable which takes string as identifier, otherwise result 
in unexpected behaviour.

(for enumerate type)
if we pass iden1.iden2, we will give an error ER_WRONG_TYPE_FOR_VAR
(Incorrect argument type to variable).