useradd, userdel, usermod: move from common to linux directory

pages/linux/usermod.md

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+# usermod
+
+> Modifies a user account
+
+- Change a user's name
+
+`usermod -l {{newname}} {{user}}`
+
+- Add user to supplementary groups (mind the whitespace)
+
+`usermod -a -G {{group1,group2}} {{user}}`
+
+- Create a new home directory for a user and move their files to it
+
+`usermod -m -d {{/path/to/home}} {{user}}`