few compile time warnings removed

few compile time warnings removed

Few small changes to remove some compile time warnings:

- we are not initializing the qemu_alarm_timer pointer in
init_timer_alarm, so I am adding a NULL initialization;

- in the headers include code, we are doing something like:

/* BSD stuff */
/* linux stuff */
/* sun stuff */

that works most of the times but it fails when you try to compile qemu
on mini-os, that is posix but it doesn't define either __sun__ or
__linux__, so I am changing it to:

/* BSD stuff */
/* linux stuff */
/* sun stuff */

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>