Fix function signatures

[MHenderson]

Warning: beside:
  function(picture, ...)
beside.grob:
  function(p, q, m, n)
rot:
  function(picture, ...)
rot.grob:
  function(g, angle)
above:
  function(picture, ...)
above.grob:
  function(p, q, m, n)
flip:
  function(picture, ...)
flip.grob:
  function(g)
plot:
  function(x, ...)
plot.picture:
  function(picture)
See section ‘Generic functions and methods’ in the ‘Writing R Extensions’ manual.