Add <? input redirection

This tries to open the given file to use as stdin, and if it fails,
for any reason, it uses /dev/null instead.

This is useful in cases where we would otherwise do either of these:

```fish
test -r /path/to/file
and string match foo < /path/to/file

cat /path/to/file 2>/dev/null | string match foo
```

This both makes it nicer and shorter, *and* helps with TOCTTOU - what if the file is removed/changed after the check?

The reason for reading /dev/null instead of a closed fd is that a closed fd will often cause an error.

In case opening /dev/null fails, it still skips the command.
That's really a last resort for when the operating system
has turned out to be a platypus and not a unix.

Fixes #4865

(cherry picked from commit df8b9b7095)

tests/checks/redirect.fish

@@ -142,3 +142,25 @@ echo "/bin/echo pipe 12 <&12 12<&-" | source 12<&0
 echo foo >/bin/echo/file
 #CHECKERR: warning: An error occurred while redirecting file '/bin/echo/file'
 #CHECKERR: warning: Path '/bin/echo' is not a directory
+
+echo foo <?nonexistent
+#CHECK: foo
+echo $status
+#CHECK: 0
+
+read -l foo <?nonexistent
+echo $status
+#CHECK: 1
+set -S foo
+#CHECK: $foo: set in local scope, unexported, with 0 elements
+
+set -l fish (status fish-path)
+$fish --no-config -c 'true <&?fail'
+#CHECKERR: fish: Requested redirection to '?fail', which is not a valid file descriptor
+#CHECKERR: true <&?fail
+#CHECKERR: ^~~~~~^
+
+$fish --no-config -c 'true <?&fail'
+#CHECKERR: fish: Expected a string, but found a '&'
+#CHECKERR: true <?&fail
+#CHECKERR: ^