chsh, chpass: add pages; pages/linux/chsh: update page (#12160)

* chpass, chsh: add page for {Free,Net,Open}BSD and update common page

* pages.*BSD: add aliases for chpass

* chpass: add page for osx

pages/openbsd/chfn.md

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+# chfn
+
+> This command is an alias of `chpass`.
+
+- View documentation for the original command:
+
+`tldr chpass`

pages/openbsd/chpass.md

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+# chpass
+
+> Add or change user database information, including login shell and password.
+> See also: `passwd`.
+> More information: <https://man.openbsd.org/chsh>.
+
+- Set a specific login shell for the current user interactively:
+
+`doas chsh`
+
+- Set a specific login [s]hell for the current user:
+
+`doas chsh -s {{path/to/shell}}`
+
+- Set a login [s]hell for a specific user:
+
+`doas chsh -s {{path/to/shell}} {{username}}`
+
+- Specify a user database entry in the `passwd` file format:
+
+`doas chsh -a {{username:encrypted_password:uid:gid:...}}`

pages/openbsd/chsh.md

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+# chsh
+
+> This command is an alias of `chpass`.
+
+- View documentation for the original command:
+
+`tldr chpass`