fix undefined usage of posix `basename()` passing `const char *`

[GitMensch]

per Linux Manpage:

There are two different versions of basename() - the POSIX version described above, and the GNU version, which one gets after

   #define _GNU_SOURCE         /* See feature_test_macros */
   #include <string.h>

The GNU version never modifies its argument, and returns the empty string when path has a trailing slash, and in particular also when it is "/". There is no GNU version of dirname().

With glibc, one gets the POSIX version of basename() when <libgen.h> is included, and the GNU version otherwise.

The inclusion of <string.h> to use basename() also breaks building outside of glibc, as seen in #4009's build log in TraceStream.cc.
If we keep using it, then we should explicit use the posix version by #include <libgen.h> in both current uses (only C++, in C we use a hard `strrchr(exe_image, '/').

So... What should it be (I'll do the PR):

• use posix version with a temporary copy of the c_str/data to a buffer and including the header
• use a C++ version with
  • a hard path.substr with path.find_last_of('/') + 1 in both places
  • a wrapper function base_name which allows also files without a path [not checking for nullptr as that is up to the caller], either defined in both files or a util header
• something else (note: using fs::path(exe_path).filename().string() won't work as the Wiki mentions RHEL7 and Ubuntu 14 - both don't have a compiler new enough installed per default; side note: I did not found an explicit minimal compiler version in the Wiki, but guess there must be one and the ancient GCC4.8/clang 3.4 distributed by those systems' package managers are possibly not working)

[rocallahan]

We require C++17. The oldest Ubuntu we support is 20.04 LTS. The oldest RHEL we support is RHEL9. Feel free to update the wiki...

Converting to the POSIX basename sounds good.

[GitMensch]

I'll do both - feel free to assign this issue to me.