Cache essential standard option definitions

In a lot of place in the code we fetch the definition of a standard option. This raises two points:

• it is not very efficient even the number of standard options is both small and bound for v4
• each time it is not possible to assume the pointer to the option defition is not null

The idea is to create some static methods in lindhcp++ returning a static pointer to the definition. The initialization code checks that there is no missing definitions. Both more efficient and safer...