Print help that was asked for to stdout (closes #179)

--help, -h and the help subcommand now print to stdout and exit 0, so
the help can be paged and searched. The usage text of a usage error
stays on stderr with exit 2, as does help for an unknown subcommand
("sfdupes help scna"). A failed write of the help exits 1.

Cobra prints help to the command's output writer, which stays stderr,
drops a failed write, and returns no error after help. So run replaces
the help function and the help subcommand: the help goes into a buffer,
then to stdout, and the error sets the exit code.

README "Settings" and goal 4 say so.

Model: opus-5-5

README.md

@@ -253,8 +253,9 @@ In order:
    `trees` check only that each copy they name still exists, and leave out a
    copy that is gone. `scan` is built to run from `cron`. The reports run at any
    time, against what the database holds.
-4. **Keep the streams apart.** Everything on stdout is machine-readable data.
-   Everything for a human reader, such as progress and warnings, goes to stderr.
+4. **Keep the streams apart.** stdout holds only what was asked for:
+   machine-readable data, or help. Everything else for a human reader, such as
+   progress and warnings, goes to stderr.
 
 ### Subcommands
 
@@ -324,12 +325,13 @@ The reference below is also the program's specification.
   "Operands").
 - `--version`, `-v` prints `sfdupes VERSION` (see "Install") to stdout and
   exits 0. It writes nothing to stderr.
-- `--help`, `-h`, alone or after a subcommand, prints the help text to stderr
-  and exits 0. It writes nothing to stdout. The help of `sfdupes` and of `scan`
-  names the database. The help of `report`, `trees`, `info` and `dwim` says what
-  each prints. `sfdupes scan --help` and `sfdupes dwim --help` also list the
-  default excludes of the system they run on, and the filesystem types the walk
-  never records anything on.
+- `--help`, `-h`, alone or after a subcommand, prints the help text to stdout
+  and exits 0. It writes nothing to stderr. `sfdupes help` and
+  `sfdupes help COMMAND` do the same. The help of `sfdupes` and of `scan` names
+  the database. The help of `report`, `trees`, `info` and `dwim` says what each
+  prints. `sfdupes scan --help` and `sfdupes dwim --help` also list the default
+  excludes of the system they run on, and the filesystem types the walk never
+  records anything on.
 
 ### Duplicate detection