main/cryptsetup: fixup man pages which use the .so directive

The crpytsetup-doc package ships several man page which just consist of an .so directive for referring to another man page. This directive is not supported by mandoc [1]. As such, opening man pages such as cryptsetup-create(8) does not work presently.

The recommended solution by upstream is to replace such man pages with a symlink. This commits adds a shell oneliner which does exactly that.