[krbdev.mit.edu #6705] plugins don't build on HP-UX

[Arlene Berry via RT]

Until recently we didn't build any of them but now we're building the
pkinit one.  The problem is that it tries to build them with a ".so"
extension but it needs to be ".sl" for HP-UX.  I first ran into it when
I was merging in 1.7 and never got around to reporting it.  There was a
new plugin that we hadn't turned off which had the same problem so I
suspect it's all of them.