common/*: update pages of POSIX special Built-In utilities (#11858)

* eval, exec, exit, export, readonly, set, shift, trap: use Open Group links

* exec, export, trap: split Linux and POSIX

pages/linux/trap.md

@@ -0,0 +1,21 @@
+# 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}}`