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/common/exec.md

@@ -1,20 +1,8 @@
 # exec
 
 > Replace the current process with another process.
-> More information: <https://linuxcommand.org/lc3_man_pages/exech.html>.
+> More information: <https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#exec>.
 
 - 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}}`