mesg: split util-linux and POSIX pages

[vitorhcl]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The PR title conforms to the recommended templates.

For #11827 and #9612.

[tldr-bot]

Hello! I've noticed something unusual when checking this PR:

• The page pages/linux/mesg.md already exists in the common directory.

Is this intended? If so, just ignore this comment. Otherwise, please double-check the commits.

[sebastiaanspeck]

The pages seem to differ in command descriptions, but the main description does not differ. It looked to me like the common-page would do the same as the linux-page.