gdb: update to 7.6
Make it possible to enable vbox extension pack easily from nixpkgs config

Signed-off-by: Shea Levy <shea@shealevy.com>
Add doxypy nixpkg

Signed-off-by: Shea Levy <shea@shealevy.com>
sqlite 3.6.23.1: Remove (unused)
DBI: Update to 1.625
sqlite: Update to 3.7.16.2
sqlite: Don't include the last component of the version in the filenames

SQLite point releases are conservative, so we don't need to refer to
SQLite versions at this level of precision.
sqlite: Make 3.7.16 the default again

I've fixed the Hydra problem and the DBIx::Class issue is only a
problem in its test suite, apparently.
nixUnstable: Update to 1.5.2pre3091_772b709
mkvtoolnix: upgrade to 6.2.0
linux-3.8: upgrade to 3.8.10
linux-3.9: upgrade to 3.9.0
linux-3.9: no longer marked as RC, updated "latest" reference.