Polish: revised translation, build pl.txt in CP852 instead of Mazovia;

[314TeR]

This PR updates the Polish translation and changes the codepage used for the Polish language file.

1. Codepage: Mazovia → CP852

nls/pl.txt is currently generated with utf8tocp maz. FreeDOS sets up Polish with
DISPLAY + MODE CON CP PREPARE/SELECT=852, so after installation the console uses CP852
and FDISK shows garbled Polish characters. The installer is affected too, because FDI runs
FDISK after SETLANG has already switched to CP852.

This PR changes maz to 852 in Makefile and Makefile.gcc. The source file
(pl_utf8.txt) stays UTF-8, so a Mazovia build is still one argument away if another
distribution needs it.

2. Revised translation

Based on Mateusz Viste's 2023 translation (thank you!), reviewed line by line:

• terminology aligned with the Polish MS-DOS/Windows FDISK: partycja podstawowa DOS,
  partycja rozszerzona DOS, dysk logiczny DOS, partycja spoza DOS, dysk twardy, MB;
• menu items in the imperative mood (as in the other FreeDOS Polish translations);
• typos fixed, missing translations added (30.10–30.13);
• table headers (9.10–9.12, 10.10–10.12, 10.101) aligned with the column positions
  used in ui.c / display.c;
• no line longer than 79 characters; Y/N keys (250.0/250.1) stay TN/tn.

Testing

Built FDISK.LNG with this Makefile change and tested FDISK 1.4.4 in QEMU on FreeDOS T2610
(installed in Polish, CP852): main menu, creating primary/extended partitions and logical
drives, set/remove active partition, the "no active partition" warning, MBR menu,
/INFO, /STATUS, /?. Screenshots attached.

Not tested on real hardware yet.