Adding log4j agent support

This adds the log4j domain to the public ABI and "-l, --log4j" option in
the lttng command line tool to be able to use it.

Note that at this commit, the JUL default channel and event name are
used for the log4j domain. That might become specific to the domain in
the long run.

Signed-off-by: David Goulet <dgoulet@efficios.com>