Commit a91bf6d88 (builtin.c: builtin_source now checks that its argument is
a file., 2005-12-16) fixed an infinite loop for commands like "source /"
where the argument is a directory.
It did so by erroring out early unless the filename argument is a regular file.
This is too restrictive; it disallows reading from special files like /dev/null
and fifos.
Today we get a sensible error without this check, so remove it.
