New API: lttng_ust_init_thread() for async-signal tracing

New API: lttng_ust_init_thread() for async-signal tracing

LTTng-UST uses Global Dynamic model TLS variables rather than IE
model because many versions of glibc don't preallocate a pool large
enough for TLS variables IE model defined in other shared libraries,
and causes issues when using LTTng-UST for Java tracing.

Because of this use of Global Dynamic TLS variables, users wishing to
trace from signal handlers need to explicitly trigger the lazy
allocation of those variables for each thread before using them.
This can be triggered by calling lttng_ust_init_thread().

Hide the public symbols for ring buffer and counter clients init/exit.
Expose new symbols for init/exit of all ring buffer and counter clients
instead. Those new symbols are only used by liblttng-ust-ctl, and are
thus in an internal header.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I389be1db96e5ff1e9a116311b8adf8676c9752cb