Add an MLINK for atf_check(1) -> atf-check(1)

This is being done to make the documentation for atf-check(1) easier to find/more
intuitive for new users, because atf_check is the atf-run(1) shell version of the
standalone atf-check(1) command, which is used in atf-sh(3) test programs.

MFC after: 1 week
Sponsored by: Dell EMC Isilon