Write encrypt and decrypt output only once they succeed (closes #143)

`secret encrypt` and `secret decrypt` emptied the file named by
`--output` before reading any input, so a failure left it empty, and a
file given as both `--input` and `--output` was emptied before it was
read. They now write into a temporary file of mode 0600 next to it, or
next to the file it is a symbolic link to, and rename that over it only
on success, with the new `secret.WriteFileAtomicFunc`. An output file
that cannot be written still fails. Ctrl-C, SIGTERM or a hangup removes
the temporary file. A device or pipe, such as `/dev/null`, is written to
directly.

Judgement calls: the device and pipe case; a command started with
`nohup` still ignores a hangup.

Model: opus-5-5

README.md

@@ -283,6 +283,19 @@ generates a new Age key.
 
 Decrypts data using an Age key stored as a secret.
 
+Both write the result to a new file next to `--output` and rename it over
+`--output` only once they have succeeded, so a failed command leaves an existing
+`--output` as it was and creates none, and `--input` and `--output` can be the
+same file. When `--output` is a symbolic link, the file it points to is
+replaced, or created if it does not exist yet, and the link kept; the new file
+is then made next to that file. The directory the new file is made in must be
+writable. The new file has mode 0600 and belongs to the user running the
+command, whatever mode and owner the file it replaces had, so a hard link to the
+old file keeps the old content. Ctrl-C, SIGTERM or a hangup removes the new
+file; a command killed any other way, such as with SIGKILL, leaves it there,
+named `.tmp-` and a number. A device or a pipe, such as `/dev/null`, is written
+to directly.
+
 ## Storage Architecture
 
 ### Directory Structure