Add LTTng-UST logger plugin

Created by: eepp

The following patches add an LTTng-UST titan.core logger plugin.

LTTng-UST is a user space tracer for Linux. The logger plugin dynamically loads an LTTng-UST tracepoint provider (separate, installed shared object) which contains the code needed to serialize the log statement payload as LTTng (CTF) event records. This is the standard workflow with LTTng-UST. This code is generated from the tracepoints defined using the LTTng-UST macros in loggerplugins/LTTngUSTLogger/tp.hh.

The logger plugin is only built if the LTTNGUSTLOGGER variable is set to yes in Makefile.cfg (defaults to no). The build requires that the LTTng-UST header files and libraries be found (the CPPFLAGS and LDFLAGS variables can be used if the project is installed outside the default search directories).

A test is added (regression_test/LTTngUSTLogger) to load the plugin and use it minimally. It is based on regression_test/junitlogger. The first two commits can be merged if this is what's done usually in the titan.core project (feel free to ask).

The logger plugin is documented in usrguide/referenceguide.doc (new section 7.2.2.4).