Fix handling of non-ASCII characters in DRAW and paths on Unix

Let OSD_Path::IsValid() always return Standard_True on Unix too.
Users may want to use filenames with non-ASCII characters. There is no reliable way to detect invalid filenames; it is bette to not perform any check and let the system handle errors.

Drop special handling of non-ASCII characters in DRAWEXE interactive mode.
On Unix, input lines were converted into Unicode codepoints, and
stored into a TCollection_AsciiString.  Non-ASCII characters
were thus replaced by garbage values (codepoint modulo 256).
    
When run in non-interactive mode via the -c flag, arguments
are processed as is (see Draw_Appli), do the same thing in
interactive mode.