The goal of this internal refactor is to fix some bugs in cancelation
and allow group tasks to clean up their own resources eagerly. The
latter will become a guarantee of the `std.Io` interface, which is
important so that groups can be used to "detach" tasks.

This commit changes the API which POSIX system calls use internally (the
functions formerly called `beginSyscall` etc), but does not update the
usage sites yet.