fixup! Instead of creating Cygwin symlinks, use deep copy by default

In Cygwin v3.5.6, the `isdevice()` method was dropped, and the commit
message of 002aad0 (Cygwin: path_conv: simplify, rearrange, rename
combined device checks, 2025-01-21), which is the commit that dropped
it, explains:

    Drop the isdevice() check in favor of a new isondisk() check.

On the face of it, the message is clear: we should use `isondisk()`
instead of `isdevice()`.

As it turns out, MSYS2's code that pretends to create a symbolic link
upon `ln -s` but instead makes a deep copy used to call `isdevice()`:

	if (!src_path.isdevice () && !src_path.is_fs_special ())
		// do a deep copy and return

	// otherwise fall back to creating a `.lnk` file

A deep dive into the issue reveals that in this instance, the condition
is actually equivalent to:

	if (!src_path.isspecial() ||
	    (src_path.isfifo() && !src_path.isondisk()))

Further study (by trying it out) reveals that replacing this condition
with `src_path.isondisk()` fails to produce the expected result. Here is
the expected result, which we receive whether we use MSYS2 runtime
v3.5.5 or the version with `src_path.isspecial()`:

	$ mknod foo c 1 3
	$ ln -s foo foolnk
	$ ls -l foo*
	crw-rw-rw- 1 x None 1, 3 Jan 27 10:43 foo
	lrwxrwxrwx 1 x None    3 Jan 27 10:43 foolnk -> foo

and with `isondisk()`, this happens instead:

	$ mknod foo c 1 3
	$ ln -s foo foolnk
	$ ls -l foo*
	crw-rw-rw- 1 x None  1, 3 Jan 27 10:41 foo
	-r--r--r-- 1 x None   126 Jan 27 10:41 foolnk

Further experimentation reveals that FIFOs are handled without bothering
with the `isfifo() && !isondisk()` condition, merely testing
`!isspecial()` is enough (and recapitulates the behavior of v3.5.5):

	$ mkfifo fifo
	$ ln -s fifo fifolnk
	$ ls -l fifo fifolnk
	prw-rw-rw- 1 None 0 Jan 27 20:44 fifo
	lrwxrwxrwx 1 None 4 Jan 27 20:44 fifolnk -> fifo

Therefore, let's just go with the sweet-and-simple `!isspecial()`.

This commit will live un-autosquashed in the `msys2-3.5.6` branch to
allow for all of the above to be documented in the commit history.

Helped-by: Jeremy Drake <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

winsup/cygwin/path.cc

@@ -2320,7 +2320,7 @@ symlink_worker (const char *oldpath, path_conv &win32_newpath, bool isdevice)
 		  set_errno (src_path.error);
 		  __leave;
 		}
-	      if (!src_path.isdevice () && !src_path.is_fs_special ())
+	      if (src_path.isspecial ())
 	        {
 		  /* MSYS copy file instead make symlink */