MEDIUM: sample/http_proto: Add new type called method

MEDIUM: sample/http_proto: Add new type called method

The method are actuelly stored using two types. Integer if the method is
known and string if the method is not known. The fetch is declared as
UINT, but in some case it can provides STR.

This patch create new type called METH. This type contain interge for
known method and string for the other methods. It can be used with
automatic converters.

The pattern matching can expect method.

During the free or prune function, http_meth pettern is freed. This
patch initialise the freed pointer to NULL.