Rewrites lttng-ctl's set_session_daemon_path

Rewrites lttng-ctl's set_session_daemon_path

This fifth patch rewrites lttng-ctl's set_session_daemon_path() to avoid
duplicating snippets of code. It also fixes the snprintf return value
test so the code works with both GNU C < 2.1 and >= 2.1. With GNU C <
2.1, snprintf returns -1 if the target buffer is too small; with GNU C
>= 2.1, snprintf returns the required size (excluding the closing null)
under the same conditions.

Signed-off-by: Daniel U. Thibault <daniel.thibault@drdc-rddc.gc.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>