age -o follows a symlink and writes a pipe or device directly (closes #59)

age encrypt -o and age decrypt -o always wrote a new file beside the
named path and renamed it over that path, which replaced a symlink, a
named pipe or a device such as /dev/null with a regular file and made
-o /dev/stdout fail. The path is now looked at without following a
final symlink: a missing path or a regular file is replaced by rename
as before, a symlink gets the same treatment for what it points at,
and anything else is written to directly, without catching signals.
A symlink that points at nothing is refused. The README says so, and
that a replaced file has mode 0600.

Model: opus-5-5

internal/cli/cli.go

@@ -85,7 +85,8 @@ func init() {
 // signals to clean up first: "ssh to" and "ssh install" while they
 // have ssh or sftp running, so the child ends and their own cleanup
 // still runs, and "age encrypt -o" and "age decrypt -o" while they
-// write, so the unfinished file is removed.
+// write a new file to rename over the named one, so the unfinished file
+// is removed.
 func Main() int {
 	err := Root().Execute()
 	if err == nil {