pflog: Modernize packet parsing

pflog: Modernize packet parsing

Enable ND_LONGJMP_FROM_TCHECK and remove a 'trunc' label.
Add MAX_PFLOG_HDRLEN and use it for a test.
Use ND_ICHECK_U() in length tests and add an 'invalid' label.

Don't check the truncation with ND_TCHECK_SIZE(hdr), because the
sizeof(hdr) depend on the OS. Use ND_TCHECK_LEN(hdr, hdrlen).
Increment ndo_ll_hdr_len only in non-truncation case.