mesg: split util-linux and POSIX pages (#12167)

pages/linux/mesg.md

@@ -0,0 +1,21 @@
+# mesg
+
+> Check or set a terminal's ability to receive messages from other users, usually from the `write` command.
+> See also `write`, `talk`.
+> More information: <https://manned.org/mesg.1>.
+
+- Check terminal's openness to write messages:
+
+`mesg`
+
+- Disallow receiving messages from other users:
+
+`mesg n`
+
+- Allow receiving messages from other users:
+
+`mesg y`
+
+- Enable [v]erbose mode, printing a warning if the command is not executed from a terminal:
+
+`mesg --verbose`