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

README.md

@@ -250,11 +250,17 @@ identity's own recipient, plus any given with `--to`, so the same mnemonic can
 always decrypt what it encrypted. Output goes to `-o` or standard output;
 `--armor` writes the text form. Nothing is written except the output.
 
+A regular file already at the `-o` path is replaced, and the new file has mode
+`0600`. A symlink there is followed, and what it points at is treated the same
+way, so the link keeps pointing where it did; a symlink that points at nothing
+is refused. A named pipe or a device, such as `/dev/null` or `/dev/stdout`, is
+written to directly.
+
 ### `keyfunc age decrypt [-n N] [-o <file>] [<file>]`
 
 Decrypts the file (or standard input) with the derived identity. Output goes to
-`-o` or standard output. If the identity is not one of the recipients, the tool
-says so and exits with status 1.
+`-o`, which is treated as for `encrypt`, or standard output. If the identity is
+not one of the recipients, the tool says so and exits with status 1.
 
 ## Derived mnemonics: `keyfunc mnemonic`
 
@@ -291,15 +297,17 @@ passes through `ssh`'s own exit status.
 
 SIGINT, SIGTERM and SIGHUP end any command at once, at the mnemonic prompt too,
 with the status a shell gives a program killed by that signal (130 for SIGINT).
-While `age encrypt -o` or `age decrypt -o` is writing the file, the signal makes
-it remove the unfinished file, leave a file already at the named path as it was,
-and exit with status 1. That holds for a signal that has reached `keyfunc` when
-its input ends; a later one leaves the whole file in place. Ctrl-C on a pipeline
-ends the input at the same moment, and on Linux `keyfunc` sees the signal first,
-though no system promises that. While `ssh to` or `ssh install` has `ssh` or
-`sftp` running, the signal ends that program instead, the tool removes its agent
-socket or working files, and it exits with status 1, or for `ssh to` with
-`ssh`'s own status if `ssh` reported one.
+While `age encrypt -o` or `age decrypt -o` is writing a new file or replacing a
+regular one, the signal makes it remove the unfinished file, leave a file
+already at the named path as it was, and exit with status 1. That holds for a
+signal that has reached `keyfunc` when its input ends; a later one leaves the
+whole file in place. Ctrl-C on a pipeline ends the input at the same moment, and
+on Linux `keyfunc` sees the signal first, though no system promises that. A
+named pipe or a device at the `-o` path is written to directly, and the signal
+ends the tool there as it ends any other command. While `ssh to` or
+`ssh install` has `ssh` or `sftp` running, the signal ends that program instead,
+the tool removes its agent socket or working files, and it exits with status 1,
+or for `ssh to` with `ssh`'s own status if `ssh` reported one.
 
 ## Entrypoints