pwgen, sshuttle, trap, tree: move to common (#6551)

pages/common/trap.md

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+# trap
+
+> Automatically execute commands after receiving signals by processes or the operating system.
+> Can be used to perform cleanups for interruptions by the user or other actions.
+> More information: <https://manned.org/trap>.
+
+- List available signals to set traps for:
+
+`trap -l`
+
+- List active traps for the current shell:
+
+`trap -p`
+
+- Set a trap to execute commands when one or more signals are detected:
+
+`trap 'echo "Caught signal {{SIGHUP}}"' {{SIGHUP}}`
+
+- Remove active traps:
+
+`trap - {{SIGHUP}} {{SIGINT}}`