1. Moved AUTOEXEC.BAT support to a separate file, reworked the code to use modern C++ features
2. The generated Z:\AUTOEXEC.BAT should be more aesthetic now
3. [autoexec] section is now UTF-8 aware

NOTE: This PR will need a small (trivial) adaptation after #2422 is merged, due to renamed UTF8_GetCodePage and UTF8_RenderForDos. When both are merged, the autoexec-related problems mentioned on Discord by @dreamer should be resolved.

A quick&dirty test - open dosbox-staging.conf in your favorite editor, add the following lines to the [autoexec] section, and save it as UTF-8 file (this is crucial!):

@ECHO.
@Z:\KEYB.COM us 437 > NUL
@ECHO  KEYB us 437  : ĄĘŁĆŃÓŚŹŻ
@ECHO.
@Z:\KEYB.COM lv 1117 > NUL
@ECHO  KEYB lv 1117 : ĄĘŁĆŃÓŚŹŻ
@ECHO.
@Z:\KEYB.COM pl 667 > NUL
@ECHO  KEYB pl 667  : ĄĘŁĆŃÓŚŹŻ
@ECHO.
@Z:\KEYB.COM pl 668 > NUL
@ECHO  KEYB pl 668  : ĄĘŁĆŃÓŚŹŻ
@ECHO.

Afterwards:

1. Start DOSBox and check (using KEYB commands above) that it printed out the text according to the code page set at given moment.
2. Switch between different code pages and type type Z:\AUTOEXEC.BAT commands - check that DOSBox always adapts the content of the AUTOEXEC.BAT to the currently active code page.