Remove the paragraph about writing to the Free Software Foundation's
mailing address. The FSF has changed addresses in the past, and may do
so again. In 2021 this is not useful, let's rather have a bit less
boilerplate at the start of source files.
Implemeted with a Galois LFSR for speed and flexibility compared to Fibonacci version.
Aliases for three popular PRBS' are added for convenience - PRBS9, PRBS15 and PRBS64.
Note that we can't test PRBS64 completely, because the sequence is too long to