Include rpl.h in the tarball.
struct newesp is required regardless of whether we have libcrypto or not.
Various fixes.

Get rid of zero-length arrays; not all compilers support them.

Get rid of __attribute__((packed)); not all compilers support it, so we
can't depend on it.

Use explicit constants for sizes, just in case some compilers pad
structures.

Add some length checks.

Make some ND_TTEST2 calls ND_TCHECK2 calls instead, so they report
truncation if they fail.

Constify some pointers.
add a decoder for Loopback/CTP

tcpdump used to print an empty line for a Loopback (CTP) packet, which
many Cisco switches send by default every 10 seconds. This commit adds
a decoder for the protocol and a test case, which uses the sample
capture from Wireshark wiki (configuration_test_protocol_aka_loop.pcap).