fix PGM "TSDU Length" field byte order handling

"TSDU Length" comes in network byte order on wire and requires
respective conversion before the use.
add a test case for PGM
reuse existing PGM decoder for EPGM too

EPGM stands for UDP-encapsulated PGM (the original PGM is
IP-encapsulated), which has no assigned UDP port number. Thus the
decoding can be done only by means of -T option, which now accepts
"epgm" protocol type. There is a test case for EPGM now.