Multiline-statement and multi-statement for pgbench custom script.

Hi, this is a spin-off patch from Fabien COELHO's
backslash-continuations.

The major concept of this patch is making usage of psql's scanner
to get rid of home-grown scanner of pgbench to make
multi-statement feature available for pgbench custom scripts.

This patch does the following things.

- Modify psqlscan.l so that unnecessary functions of it can be
masked when used in other modules like pgbench.

- Modify pgbench to use psqlscan.l so that the following features
available in pgbench.

- multi-statement in custom scripts.
- natural continuation of SQL statements in costom scripts.
- backslash-continuation for pgbench metacommands in costom scripts.

The patch consists of following files.

- 0001-Prepare-to-share-psqlscan-with-pgbench.patch
Modifies psqlscan.l in psql as the preparation.

- 0002-Make-use-of-psqlscan-for-parsing-of-custom-script.patch
Modifies pgbench to use psqlscan.l.

- 0003-Change-MSVC-Build-script.patch
Modify MSVC build script.

- 0004-Change-the-way-to-hold-command-list.patch
Get rid of double-format of internal command list.
This changes the way of holding command list to linked list
totally.

regards,