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 better
to not perform any check and let the system handle errors.
Drop special handling of non-ASCII characters in 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.
Drop also ASCII checks on various OSD_Path::Set*() methods on Unix

Non-ASCII names are valid, and these checks are not performed on Windows.
Add test OCAFExport_test to check problems with non-ASCII paths
Merge pull request #363 from tpaviot/review/non-ASCII-paths

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