This sets the range of the first automatically assigned PMT PID or elementary
stream PID parameters to [0x20, 0x1ffa]. You can still assign manually a PID
for a stream using AVStream->id in the wider [0x10, 0x1ffe] range as specified
by ISO13818-1. But since DVB and ATSC both reserves some PIDs, let's not allow
them to be automatically assigned.

Also make sure that assigned PID numbers are valid and fix the error message
for the previous PID collision checks.
Signed-off-by: Marton Balint <cus@passwd.hu>