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/exec.md

@@ -0,0 +1,20 @@
+# exec
+
+> Replace the current process with another process.
+> More information: <https://linuxcommand.org/lc3_man_pages/exech.html>.
+
+- Replace with the specified command using the current environment variables:
+
+`exec {{command -with -flags}}`
+
+- Replace with the specified command, clearing environment variables:
+
+`exec -c {{command -with -flags}}`
+
+- Replace with the specified command and login using the default shell:
+
+`exec -l {{command -with -flags}}`
+
+- Replace with the specified command and change the process name:
+
+`exec -a {{process_name}} {{command -with -flags}}`