Cleanup: Cast poll() return value to void

*** CID 1019969:  Unchecked return value from library  (CHECKED_RETURN)

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Cleanup: Mixed enums used for return code in send_sessiond_channel()

*** CID 1132649:  Inferred misuse of enum  (MIXED_ENUMS)

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Cleanup: No check of the return value of lttng_ht_del()

*** CID 1230590:  Unchecked return value  (CHECKED_RETURN)

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Cleanup: Unchecked close() return value in utils_create_lock_file()

*** CID 1226188:  Unchecked return value  (CHECKED_RETURN)

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