Slurping directories makes sense on some OSes

Some OSes (eg. FreeBSD) follow the traditional Unix approach of treating
directories like files, and so slurp()ing directories on those platforms
is allowed

S32-setting-library/IO.pod

@@ -193,7 +193,8 @@ scope by declaring:
 Slurps the entire file into a C<Str> (or C<Buf> if C<:bin>) regardless of context.
 (See also C<lines>.)
 
-The routine will C<fail> if the file does not exist, or is a directory.
+The routine will C<fail> if the file does not exist, or if the file is a
+directory and the platform does not support opening a directory like a file.
 
 =item spurt