how to reading 0 byte files properly

[Binbin YE <phantom2501@gmail.com>]

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Hi devs!

I've been trying using emacs to view some files mounted by FUSE. Sadly file
attr in the system says the file size is 0 bytes, just like files under
/proc

emacs can open the file but only can open it partially. I've checked the
code to handle find-file and narrow down to

```c
/* emacs/src/fileio.c:4587 */

if (seekable || !NILP (end))
  total = end_offset - beg_offset;
else
  /* For a special file, all we can do is guess.  */
  total = READ_BUF_SIZE;
```
Judging from the code, it assume the total size would be READ_BUF_SIZE

which seems to be not a very big number ( or not, I'm not sure)

```c
/* emacs/src/fileio.c:3692 */

enum { READ_BUF_SIZE = MAX_ALLOCA };
```

```h
/* emacs/src/lisp.h:5300 */

enum MAX_ALLOCA { MAX_ALLOCA = 16 * 1024 };
```

since vscode, vim, cat, and less can read this file properly, I would like
to clarify whether it is a bug to fix or there another way to open a big
file like this (say the actual size is 2MB~ but showing as 0 byte on stat)

Best,

Binbin

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