Axe the broken fpio support for good

Axe the broken fpio support for good

- The whole thing is extremely hacky and failing to properly take
  fundamental differences in eg fread() and read() semantics into
  account (the whole rpmio fd is confused over this: Fread() claims
  to be fread() clone but reads behave with read() semantics
  and we dont provide feof() equivalent etc), special cased all over
  the io code when vectors exist for this purpose etc (not that fpio
  is the only offender in that regard)... RIP. Nothing should miss
  this but if we'd ever want to add it back, it'd better be done
  right at least.
- This COULD silently break somebody's code if they rely on the broken
  special case semantics of fpio mode, but most likely rpm itself
  was the only thing using fpio (and relying on the behavior) and
  even then, mostly only to get a FILE stream pointer out of it.
  Now we'll just silently return a fdio descriptor, as we do for any
  unrecognized io mode... (another sigh)