The data structure for per-packet headers for Mac V5 and V6 Etherpeek

The data structure for per-packet headers for Mac V5 and V6 Etherpeek
files would put a 32-bit quantity on a 16-bit boundary without padding;
this means that many compilers will insert the padding and thus make the
structure not match what's in the file.

Instead of using a C structure, #define values for the offsets of
fields, read the header into an array of bytes, and extract values using
the offsets.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4334 f5534014-38df-0310-8fa8-9805f1628bb7