gpasswd: add page (#2480)

pages/linux/gpasswd.md

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+# gpasswd
+
+> Administer "/etc/group" and "/etc/gshadow".
+
+- Define group administrators:
+
+`sudo gpasswd -A {{user1,user2}} {{group}}`
+
+- Set the list of group members:
+
+`sudo gpasswd -M {{user1,user2}} {{group}}`
+
+- Create a password for the named group:
+
+`gpasswd {{group}}`
+
+- Add a user to the named group:
+
+`gpasswd -a {{user}} {{group}}`
+
+- Remove a user from the named group:
+
+`gpasswd -d {{user}} {{group}}`