Allow canceling of iphb wakeups by iphb_wait() with zero wakeup range

The daemon side has always assumed that using zero wait window means
the client wants to cancel the currently programmed wakeup. For some
reason it was never noticed that the client library does not make the
request unless a non-zero wait period is defined.

Do not ignore iphb_wait*() requests with zero wait period.

[libiphb] Allow canceling of iphb wakeups by iphb_wait() with zero wakeup range. Fixes JB#20629
Merge pull request #9 from spiiroin/empty_wait_cancels

Allow canceling of iphb wakeups by iphb_wait() with zero wakeup range