pg_dump: Remove "blob" terminology

For historical reasons, pg_dump refers to large objects as "BLOBs".
This term is not used anywhere else in PostgreSQL, and it also means
something different in the SQL standard and other SQL systems.

This patch renames internal functinos, code comments, documentation,
etc. to use the "large object" or "LO" terminology instead. There is no
functionality change, so the archive format still uses the name "BLOB"
for the archive entry. Additional long command-line options are added
with the new naming.