archivers/pbzip2: fix build with libc++ 19

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes archivers/pbzip2 to fail to compile with clang 19 and libc++
19, resulting in errors similar to:

/usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned char>'
  820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
      |                                          ^
./BZ2StreamScanner.h:128:25: note: in instantiation of template class 'std::basic_string<unsigned char>' requested here
  128 |         basic_string<CharType> _bz2Header;
      |                                ^
/usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
   23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
      |                             ^

This can be fixed by defining _bz2Header and _bz2HeaderZero as
std::vector<CharType> instead of std::basic_string<CharType>. It
requieres a few other changes, such as replacing operator= with
assign, and compare with equal.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

PR: 282528
Approved by: farrokhi (maintainer)
MFH: 2024Q4

(cherry picked from commit 29c80f114ea6cc60be39502339572af8c35ac440)