Fix an unicode issue with amx_SetStringOld.

[Arkshine]

Issue found by Nextra, while playing with contain native and noticed it was not working properly with string containing unicode characters. It's because of a wrong data type conversion where signed char negative values will give 0xffff* resulting failed check e.g. 0xffffc2 == c2.

This is the same issue with set_amxstring, which has been fixed.

amx_SetStringOld has actually a param to handle this situation.
Proposed fix is to use enable last param to use wchar conversion instead.

[Nextra]

This is the same bugfix done for set_amxstring in a86ca14 (direct link to change), initially reported in bug 5945.

Everything reaching plugin forwards will currently break if it contains UTF8 characters. I found this because FM_AlertMessage loglines would fail comparison with contain. Using the already existing wchar parameter should be fine.

[Nextra]

And I forgot wchar_t is platform specific. Linux builds are crashing, we need to change it to the unsigned char cast method we used for set_amxstring.