[NTG-context] simplify \setuppapersize call

[Sanjoy Mahajan]

Should \setuppapersize automatically assume the same typesetting and
printing papersizes, if no optional argument is given?  Then you could
do \setuppapersize[letter] instead of \setuppapersize[letter][letter]

Right now 

\setuppapersize[letter]
\starttext
hello
\stoptext

produces a mixture of letter and A4 dimensions:

$ texexec --pdf a > /dev/null && pdfinfo a.pdf | grep 'Page size'
Page size:      612 x 841.89 pts

Shouldn't it be either 612x792 (letter) or 595.276 x 841.89 pts (A4)?

                   tex : pdfeTeX, 3.141592-1.21a-2.2 (Web2C 7.5.4)
               context : ver: 2006.03.25 13:21
               cont-en : ver: 2006.03.25 13:21  fmt: 2006.4.11  mes: english

It might be because I once ran 'texconfig' to set the default papersize
to letter?

-Sanjoy