Changed netbuild to use POST, rather than GET.
This should alleviate some of the truncation problems we've been having, and
also is a bit cleaner looking to the client.

The applet posts to nssave.php3, which writes the ns file to /tmp/$uid-$hash.nsfile
The applet then launches beginexp/php3 in a new browser window, passing it the $hash.
It can then pass that $hash to spitnsdata.php3, or build "/tmp/$uid-$hash.nsfile"
as $nsfile (the NS file to use in expt creation.)

There are, as of yet, no file size restrictions, nor is there a mechanism to clean up
old nsfiles (which really only need to stick around for a short while, <<1hr.)