Refactor liblttng-ust-jul in liblttng-ust-agent

This is a major refactor of the initial implementation of the UST JUL
agent. The agent now support the log4j logging backend.

The agent can now be built in three different configurations:

1) Java agent with JUL support:

$ ./configure --enable-java-agent-jul

2) Java agent with Log4j support:

$ export CLASSPATH=$CLASSPATH:/path/to/log4j.jar
$ ./configure --enable-java-agent-log4j

3) Java agent with JUL + Log4j support

$ export CLASSPATH=$CLASSPATH:/path/to/log4j.jar
$ ./configure --enable-java-agent-all

To build the agent with log4j support, make sure that the log4j jar
is in your Java classpath.

The configure script will automatically detect the appropriate Java
binaries to use in order to build the Java agent.

The name of the agent jar file is now "liblttng-ust-agent.jar".
It will be installed in the arch-agnostic "$prefix/share/java" path
e.g: "/usr/share/java".

In order to support older applications using the "org.lttng.ust.jul"
package, a transitional package is built with the same name.

All applications should move to use the "org.lttng.ust.agent" package.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>