System variable "check" and "update" functions are not called on initial value.

When declaring a GLOBAL or SESSION Variable it is possible the specify "check" and/or "update" functions enabling to check the new variable value and to execute some other actions. However, if these functions are called when using the SET command, they are not called to set the initial value coming from the cnf file or the command line.

These functions should also be called on initial values. Not doing so is a risk to accept bad values and prevent to execute actions that can be required by the program.

It also prevents things such as specifying a numeric value as 512M in the SET command. The variable could be declared as "string" instead of "integer" and in the "update" function it would be possible to change the representation to an integer, the same that is done when analysing the values given in cnf file.