Change CTF Writer API headers' name to plural where appropriate

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change bt_ctf_clock's functions signature to return int (in case of error)

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix namespacing of the CTF Writer reference counting API

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix bt_ctf_stream_create's signature to return an instance instead of void

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add _END entries to byte order and string encoding enums

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix bt_ctf_writer_set_endianness' signature to report invalid arguments

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>