get offset of a member of a structure by the member name
id - structure type ID
member_name - name of structure member
returns: -1 if bad structure type ID is passed
or no such member in the structure
otherwise returns offset of the specified member.
NOTE: Union members are, in IDA's internals, located
at subsequent byte offsets: member 0 -> offset 0x0,
member 1 -> offset 0x1, etc...
long get_member_offset(long id, string member_name);