Fix: babeltrace-log packet seek

Packet seek has been removed from ctf helper headers somewhere around
Babeltrace 1.1. Since then, babeltrace-log is broken when it needs to
output a file larger than the fixed-sized packet (WRITE_PACKET_LEN
currently defined as (getpagesize() * 8 * CHAR_BIT).

Correctly handle the non-zero return values on ctf_align_pos() and
ctf_move_pos() on the dummy position copy, which are used to validate
whether we need to switch packet or not. Also add an initial packet seek
at the beginning of the file.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>