netlink: improve snl(3)

[melifaro]

Summary

• add snl_send_message() as a convenient send wrapper
• add signed integer parsers
• add snl_read_reply_code() to simplify operation result checks
• add snl_read_reply_multi() to simplify reading multipart messages
• add snl_create_genl_msg_request()
• add snl_get_genl_family() to simplify family name->id resolution
• add tests for some of the functionality

[kp]

sys/netlink/netlink_snl.h

266

cc is unhappy about the comparison between send's ssize_t and sz's size_t.