[Rd] readBin should check that its endian argument is a legal value

[Jennifer Lyon]

I think it would be helpful if readBin checked that its endian argument is
a legal value.

Why? I was reviewing some of our code and noticed that the author had
readBin(..., endian="network") and never having heard of "network", I
looked at the man page for readBin, and it hadn't heard of "network"
either. Not good.

I then looked at the R code for readBin, which has this line:
 swap <- endian != .Platform$endian

and swap is passed into the .Internal(readBin). Further use of Google
revealed that "network" is a known endian in the universe, and our code was
working by essentially a lucky chance that the data was "big" and our
current machines are "little". Really not good. I don't know enough about
endian stuff to know if it makes sense that "network" should be one of the
choices for endian for readBin (which from the documentation currently are:
"big", "little" or "swap"), but in my opinion R should have failed with the
choice of "network" in our code in the current version of R. I did
eventually find an aside in the R Data Import/Export document that
"network" is "big" so I will patch our code to be legal. Of course some
code may depend on this undocumented behavior, but I would guess that in
the vast majority of cases an illegal value really is a mistake.

Jen

> sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.3 LTS

Matrix products: default
BLAS:   /home/mbr/r-project/R-3.6.1/lib/libRblas.so
LAPACK: /home/mbr/r-project/R-3.6.1/lib/libRlapack.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] compiler_3.6.1

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