[tlbuild] tl2018 schedule a month earlier

[Karl Berry]

For 2018, Herbert and Manfred have requested that the release happen
about a month earlier. This will allow them to include the DVD with an
issue of the Komodie.

Thus, all dates would move up a month. It seems aggressive but
doable to me. Here is my tentative schedule:

19feb: sources committed, builds begin.
12mar: tlnet (and TL'17) frozen, pretest starts, CTAN updates continue.
 2apr: complete code freeze for final build, major bug fixes only.
 9apr: final updates from CTAN, final doc tweaks.
16apr: deliver TL image for TeX Collection packaging/testing.
23apr: deliver TC image for manufacturing.

This is posted on tug.org/texlive now. Let me know if any horrible
problems ... --thanks, karl.