(Torturing the Scanner): Surround the backslash-newline tests with

"#if 0", to make it less likely that we'll run into compiler bugs.
Bring back solitary \ inside comment, but add a closing comment to
work around HP C bug.  Don't test backslash-newline in C character
constant.  This should fix the input.at bug reported by Nelson
H. F. Beebe in
<http://mail.gnu.org/pipermail/bug-bison/2002-November/001893.html>.