Polish: switch strings to CP852 and update the translation

[314TeR]

This PR updates the Polish FreeCOM messages and moves them from Mazovia to CP852.

1. Codepage: Mazovia → CP852

strings/polish.lng and polish.err are encoded in Mazovia (CP991). FreeDOS sets up Polish
with CP852 (DISPLAY/MODE CON CP PREPARE), so the shell prints garbled Polish characters.
The files are now CP852, and .gitattributes gets
strings/polish.??? working-tree-encoding=CP852 (the CP991 line was commented out because
iconv has no Mazovia), so Polish is stored as UTF-8 like the other languages.

2. Translation

Based on Mateusz Viste's translation (thank you!), reviewed by a native speaker:

• 8 missing messages added (LFN, "Permission denied", corrupt EXE) and 21 untranslated ones
  translated; command help (/?) translated following the Polish MS-DOS 7 / Windows 98
  wording ("interpreter poleceń", "znak zgłoszenia");
• PROMPT_YES_NO: keys TyNn → TtNn (lower-case "t" did not work);
• polish.err: KEYS_RETRY: pP / KEYS_ABORT: eE did not match the text "(P)rzerwij,
  p(O)nów" — P retried instead of aborting. Now oO = retry, pP = abort;
• printf fixes in help text (SHIFT: %0 → %%0; FOR example);
• no help line reaches column 80; PROMPT /? and SET /? fit on one screen like in English.

Testing

Built STRINGS.DAT with fixstrs polish and the criter resource with critstrs polish +
NASM, appended them to the command.exe part of FreeCOM 0.86 (T2610) and ran it in QEMU
(CP852): all /? screens, DIR/COPY/DEL prompts, DATE/TIME, MEMORY, errors.
The XMS_Swap build has the critical error handler compiled into the executable
(xms_crit.asm includes criter.asm), so the Polish critical error text needs a full build;
not tested yet.