WL#2389: GIS: Add synonym for Length

MySQL will implement a synonym for the GIS Glength function
(Length). This task cannot be completed until the new parser 
is available.

Rationale 
--------- 
Support for the Length function is required so that MySQL can 
claim compliance with OpenGIS; see WL#2377 for full details.

Syntax
------
MySQL has implemented a GLength function, which takes one argument 
-- either a linestring value or a multilinestring value -- and returns 
a double-precision number representing the sum of the lengths of the
elements that make up the function's argument. The OpenGIS specification
calls this the Length function. For compatibility with the standard, 
MySQL must implement:
Length(linestring_value)
Length(multilinestring_value)

-- Length is a synonym for GLength. No other changes need to be made
to the function as it currently operates.


Implementation Notes
--------------------
At the time that GIS functionality was being added to MySQL, the server
already supported a Length function, for character data. Due to this,
it was necessary to rename the GIS Length function, so that there would
be no conflict. Adding the Length synonym for GLength will not be possible 
until the new parser has been implemented. Until that time, the MySQL
server is not capable of distinguishing between Length as a GIS
function and Length as a character function.