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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/52b5568432963b721698a2df1f37a0795b9611dc

Modified Files
--------------
doc/src/sgml/ref/pg_waldump.sgml | 58 ++++++++++++++---------------
src/bin/pg_waldump/pg_waldump.c | 80 +++++++++++++++++++---------------------
2 files changed, 66 insertions(+), 72 deletions(-)