poll-loop: Change poll_timer_wait() parameter from "int" to "long long".

poll-loop: Change poll_timer_wait() parameter from "int" to "long long".

Every so often I get concerned because OVS does most of its time arithmetic
in "long long int" but poll_timer_wait() takes an "int", so there is
potential for truncating a large value to a small value or a positive value
to a negative value.  That would cause excessive wakeups and possibly 100%
CPU usage.

This commit therefore changes poll_timer_wait()'s parameter type from "int"
to "long long int".  The file-scope 'timeout' variable remains type "int"
because that is the type of poll()'s timeout argument.

Factoring poll_timer_wait() into two functions is not necessary here but it
comes in handy in the following patch.