Bug 10613: calculate the GST rate correctly on the invoice page.

Test plan:
Defined a GST rate on creating an order, receive it and check that all
prices are correctly calculated.
/!\ Behavior change function of supplier parameters (Include/Don't
include tax for list prices and invoice prices)

Notes: patch tested with Bug 11755 applied first; confirmed that:
- price calculations are correct for all combinations of
  listincgst/invoiceincgst settings in the vendor record
- unitprice (aka "Actual cost") is taken into account on the
  invoice page instead of rrp/ecost, like it should.

Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 10613: (follow-up) fix typo supplierid vs booksellerid

GetInvoiceDetails returns a hashref with a key named booksellerid, not
supplierid.
The bookseller was not retrieved from the DB and the listincgst value
was always false.

Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 10613: (follow-up) fix QA issues

This patch fixes the following QA issue:

 FAIL    acqui/invoice.pl
   FAIL      valid
        Useless use of private variable in void context

Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Patch passes all tests and QA script. Specifically checked
the t/db_depenedent/Acq* tests.

A test plan could be:
0) Add a value in the gist pref (0.1 or 0.25 or something else easy).
1)
  a) Create a supplier "10613 0 0" with
    List item price includes tax: No
    Invoice item price includes tax: No
    Tax: 10%
  b) Create a supplier "10613 0 1" with
    List item price includes tax: No
    Invoice item price includes tax: Yes
    Tax: 10%
  c) Create a supplier "10613 1 0" with
    List item price includes tax: Yes
    Invoice item price includes tax: No
    Tax: 10%
  d) Create a supplier "10613 1 1" with
    List item price includes tax: Yes
    Invoice item price includes tax: Yes
    Tax: 10%
2) Create a basket for each supplier
  a) 00 List price: 10.00 (11.00 with 10% taxes)
  b) 01 List price: 10.00 (11.00 with 10% taxes)
  c) 10 List price: 10.00 (9.09 without taxes)
  d) 11 List price: 10.00 (9.09 without taxes)
  Note: Information on the basket page is shown correctly.
  If you look at the list of ordered items for the fund,
  the list price is used.
3) Create 1+ order(s) with 1+ item(s) for each basket with
   a discount and a gst value.
4) Close the baskets
5) Receive the items
   Left actual price as suggested:
   a) 00 Actual cost: 10.00
   b) 01 Actual cost: 11.00
   c) 10 Actual cost:  9.09
   d) 11 Actual cost: 10.00
   Calculations on the invoice page now all appear to be correct.
   Note: When you take a look at the 'ordered' list for the fund,
   the actual price is used as entered.
6) Go on acqui/invoice.pl?invoiceid=XX acqui/basket.pl?basketno=YY for
   each basket/invoice, click on the "Show all details" checkbox
   and verify that the values are all correct.

Calculations are exactly the same for tax registered yes and no.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>