use existing PGM decoder for UDP-encapsulated PGM

use existing PGM decoder for UDP-encapsulated PGM

The original PGM uses its own IP protocol number. "EPGM" or "PGM/UDP"
stands for UDP-encapsulated PGM, which has no assigned UDP port number
and can be decoded only by means of -T option, which now accepts "pgm"
protocol type for this purpose. There is also a sample capture of EPGM
now (similar to the one of native PGM, but produced using the "epgm://"
protocol schema) and a respective test case.