Add UnRAR 7 compatibility

[krvi]

Description:

I've been using rar2fs for years, getting it from Entware. After upgrading libunrar to 6.2.12 a couple of months ago, rar2fs stopped working for me, so I had to revert back to 6.1.3.

I noticed that UnRAR started using new types from C++11 and removed the NM constant. I looked into #186 and, over the past few days, refreshed my knowledge of C++ to address this issue.

Changes Made:

• Adapted dllext.cpp to work with new types introduced in UnRAR 7
  • In those cases, removed dependencies on the NM constant.

    Testing:

• Compiled without warnings with unrar-7.0.9.
  • Tested with unrar-7.0.9 on a multi-volume archive with filenames containing 4-byte UTF-8 characters. Also tested on a few more common / UTF-8-named archives and confirmed they worked as expected.
• Compiles without warnings with unrar-6.1.3, however, I have not tested it with this version.

[hasse69]

Thanks for the patches! Would it be possible for you to test the backwards compatibility before we merge this?