Previously glcpp would silently abort if it couldn't fstat the file being
read, (so it would work with stdin redirected from a file, but would not
work with stdin as a tty). The stat was so that glcpp could allocate
a buffer for the file content in a single call.
We now use talloc_realloc instead, (even if the fstat is
possible). This is theoretically less efficient, but quite irrelevant,
(particularly because the standalone preprocessor is used only for
testing).
This error message was missing so that the program would simply
segfault if the provided filename could not be opened for some reason.
While we're at it, we add explicit support for a filename of "-" to
indicate input from stdin.
