Add BadFdExtra core error.

Add BadFdExtra core error.

Introduce a new FdBadFd type with associated extra info struct.
Which for now just holds the fd number (no path or description).
fd_pp_Error and fd_update_extra have been updated to handle the
new type and produce xml when requested.

Rename showing_core_errors to showing_core_warning
(returns yes when the tools wants to show core errors,
-q isn't given and we aren't producing xml).

In ML_(fd_allowed) we now call VG_(maybe_record_error) to
generate a real error (that can be suppressed and shows up
in the xml output with full execution backtrace). For now
we also produce the legacy warnings when --track-fds=yes
isn't given.

Add none/tests/fdbaduse.vgtest to test the new FdBadUse
core error.

This is the first part of reporting bad fd usage errors.
We are also tracking already closed file descriptors which
should also produce errors like these. The current bad file
descriptors are just those that are negative or above the
current file limit of the process.

https://bugs.kde.org/show_bug.cgi?id=493418