Invent on_exit_nicely for pg_dump.

src/bin/pg_dump/common.c

src/bin/pg_dump/compress_io.c

src/bin/pg_dump/dumputils.c

src/bin/pg_dump/dumputils.h

src/bin/pg_dump/pg_backup.h

src/bin/pg_dump/pg_backup_archiver.c

src/bin/pg_dump/pg_backup_db.c

src/bin/pg_dump/pg_backup_directory.c

src/bin/pg_dump/pg_backup_files.c

src/bin/pg_dump/pg_backup_tar.c

src/bin/pg_dump/pg_dump.c

src/bin/pg_dump/pg_dump.h

src/bin/pg_dump/pg_dumpall.c

src/bin/pg_dump/pg_restore.c