Adding variables for segment_size, wal_segment_size and block sizes

Now that we have customizable segment sizes for heap and WAL at compilation 
time i would like to have some runtime variables to query that information 
(besides pg_controldata). I can imagine to have the following names:

segment_size: Reports heap segment size
wal_segment_size: Reports wal segment size
block_size: Available yet
wal_block_size: wal block size

I'd like to implement them if we agree on them

--  Thanks
                   Bernd