[tex-live] t;18 pretest init

[Richard M. Koch]

Folks,

The tlpretest site now has mactex-2017.pkg and mactex-basictex-2017.pkg.

Note that our support policy has changed. MacTeX now supports the last three
versions of macOS, exactly the operating systems for which Apple releases
security patches. So mactex-2017 and basictex-2017 require Yosemite, 
El Capitan, or Sierra.

The TeX Live install script has legacy binaries for older versions of 
macOS, supplied by Mojca Miklavec.

Dick Koch