Modify oiiotool --zover to take optional "zeroisinf=%d" argument.

Modify oiiotool --zover to take optional "zeroisinf=%d" argument.
Also change the underlying ImageBufAlgo::zover in the corresponding way.
When nonzero, this will interpret z=0 values as actually infinitely far
away.  Why some apps use z=0 for tmpty pixels instead of a HUGE value
for this, I don't know.  I'm talking to you, Maya.