How to upgrade s6

to 2.0.0.0

The build system has completely changed. It is now a standard ./configure && make && sudo make install build system. See the enclosed INSTALL file for details.
slashpackage is not activated by default.
shared libraries are not used by default.
skalibs dependency bumped to 2.0.0.0.
s6-supervise does not create the ./event fifodir as public. Subscriptions are restricted to users having the same gid as the s6-supervise process.
The s6-notifywhenup program is a new addition.