liburcu: new package

liburcu is a userspace RCU (read-copy-update) library that is used by
LTTng - Linux Trace Toolkit (next generation).
lttng-ust: new package

This is the user space library for LTTng. It is needed to do user space
tracing, as opposed to kernel space tracing.
lttng-tools: new package

This is the Linux Trace Toolkit. Included in this package:

Command-line client:
  lttng
Tracing daemons:
  lttng-sessiond (automatically started by lttng)
  lttng-relayd   (remote trace collection daemon)

Userspace tracing can be done by using liblttng-ust. To do kernel
tracing we also need the LTTng kernel modules.

I've added a patch that changes "/sbin/modprobe" to just "modprobe".
babeltrace: new package

Babeltrace is a command-line tool and library to read and convert LTTng
tracefiles. Give it a (binary) trace file/dir path and it will print a
human readable event log to standard out.
lttv: new package

Linux Trace Toolkit Viewer is a GUI program to visualize LTTng trace
files. (An alternative to this program is the LTTng plugin for the
Eclipse IDE.)
lttng-modules: new package

Add linux kernel modules needed to do kernel tracing with LTTng.

To make them available to lttng in NixOS, add this to configuration.nix:

  boot.extraModulePackages = [ pkgs.linuxPackages.lttngModules ];