Extend API and remove lttng_uri from lttng.h

This is a big commit but actually not adding much.

First, struct lttng_uri is removed from lttng.h and replaced by calls
using string URL. The lttng_set_consumer_url is changed to "_url" taking
an handle, a control and a data string URL. You can find the definition
in proposal doc/proposals/0004-lttng-address-api.txt.

The lttng_create_session_uri is removed and the path from the original
lttng_create_session is now used to either pass a local filesystem full
path or a string URL.

Multiple fixes in uri_parse() also especially for IPv6 where address
MUST be enclosed in brackets [] in the string representation.

The help of lttng create and enable-consumer is updated and improved.

Also, the enable-consumer command can now be used without a domain
switch (-u/-k) so the destination URL is set to the global tracing
session and on both UST and kernel session if there is an existing
consumer.

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