[bug-gv] Do not start in middle of page

[積丹尼 Dan Jacobson]

When the user does
$ gv some_document.pdf
the user is now staring at the MIDDLE of the first page, not the TOP.
Who would have thought of that... "well at least it is not the bottom of
the page" he thinks, and hits the up arrow and pretends it never
happened.

gv should open on the top, not the middle of the page. (What if when
taking pictures of our friends, we only took pictures of their belly
buttons or belt buckles, not their heads?)

Compare
$ firefox file.pdf
$ chromium file.pdf

OK I tried --nocenter. Yes, that fixes the vertical problem, but
introduces a horizontal problem!

Thus you should add
-[-][no]center[=vertical,horizontal]
at least, if not also fixing the default.

I want to open documents centered horizontally, but "nocentered"
vertically.