Since commit 290bb0dd

, where various members of GTask were converted to
a bitfield, some of the getters:
 • g_task_get_check_cancellable()
 • g_task_get_return_on_cancel()
 • g_task_get_completed()
have been returning truthy ints (zero or an arbitrary non-zero integer)
as boolean values, rather than the canonical boolean ints of 1 and 0.

This broke the `yield` statement in Vala, whose generated C code
compares `g_task_get_completed (…) != TRUE`. i.e. Whether the
`completed` field has a value not equal to 1.

Fix this by explicitly converting truthy ints to canonical boolean ints
in all getters.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

#1636