Generic linked-list filter

Implement filter_list so that nodes for which keep returns 0 are removed. Nodes kept must remain in their original order.

typedef struct node {
    void *data;
    struct node *next;
} node_t;

void filter_list(
    node_t **head,
    int (*keep)(const void *),
    void (*destroy)(void *)
);

For every removed node, call destroy on its data, free the node, and correctly update *head when the first node is removed.