timer: user-instance: add an argument for open(2) flag into API to open ALSA timer...

timer: user-instance: add an argument for open(2) flag into API to open ALSA timer character device

As long as using the created Gsource with GMainContext, the call of poll(2)
system call with infinite timeout surely returns when quit() method is
called for the context. All of GMainContext implements GWakeup with eventfd
and the call of quit() emits event via file descriptor of eventfd. This
brings wakeup from blocking when poll(2) is called with inifinite timeout.
This means that it's safe for event polling without O_NONBLOCK.

On the other hand, there are many options for file descriptors; e.g.
O_APPEND and O_CLOEXEC. For the case, glib framework has no support.

This commit adds an argument for the open flags.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>