Improve command line options for pg_waldump.

Improve command line options for pg_waldump.

Follow-up improvements for commit 127aea2a based on discussion:

* use fork name for --fork, not number
* use -R, -B as short switches for --relation, --block
* re-alphabetize the list of switches (code, --help and docs)

Suggested-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com> (fork name part)
Reviewed-by: David Christensen <david.christensen@crunchydata.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/3a4c2e93-7976-2320-fc0a-32097fe148a7%40enterprisedb.com