fix bugs, make R check happy.
s/read.image2k/p2k.read/g
s/write.image2k/p2k.write/g

(all for the hope that some day i'll have dt2k.read, i2k.read, et al.,
for data.table, image, etc., structures.)
make sure requires are made and that they work.
replace im2k.fprintf with im2k.errprintf.  this means we don't have
references to stdin, stderr in our .o files, which keeps "R CMD check"
happy.
get rid of some unused files in the R .tar.gz file.