pathchk: move to pages/common

`pathchk` is not exclusive to the Linux platforms; it's distributed with most
(if not all) BSD platforms, including...

- [FreeBSD](https://www.freebsd.org/cgi/man.cgi?query=pathchk&sektion=1)
- [macOS](https://opensource.apple.com/source/file_cmds/file_cmds-230/pathchk/pathchk.1.auto.html)