pgsql: Provide for parallel restoration from a custom format archive.

Log Message:
-----------
Provide for parallel restoration from a custom format archive. Each data and
post-data step is run in a separate worker child (a thread on Windows, a child
process elsewhere) up to the concurrent number specified by the new pg_restore
command-line --multi-thread | -m switch.

Andrew Dunstan, with some editing by Tom Lane.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        pg_restore.sgml (r1.77 -> r1.78)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_restore.sgml?r1=1.77&r2=1.78)
    pgsql/src/bin/pg_dump:
        pg_backup.h (r1.48 -> r1.49)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup.h?r1=1.48&r2=1.49)
        pg_backup_archiver.c (r1.161 -> r1.162)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_archiver.c?r1=1.161&r2=1.162)
        pg_backup_archiver.h (r1.76 -> r1.77)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_archiver.h?r1=1.76&r2=1.77)
        pg_backup_custom.c (r1.40 -> r1.41)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_custom.c?r1=1.40&r2=1.41)
        pg_backup_db.c (r1.80 -> r1.81)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_db.c?r1=1.80&r2=1.81)
        pg_backup_files.c (r1.34 -> r1.35)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_files.c?r1=1.34&r2=1.35)
        pg_backup_null.c (r1.19 -> r1.20)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_null.c?r1=1.19&r2=1.20)
        pg_backup_tar.c (r1.62 -> r1.63)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_tar.c?r1=1.62&r2=1.63)
        pg_dump.c (r1.518 -> r1.519)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.518&r2=1.519)
        pg_restore.c (r1.91 -> r1.92)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_restore.c?r1=1.91&r2=1.92)