init: add SelabelInitialize() for subcontext

Children of init that use any of the SELinux wrapper functions,
including make_dir(), mkdir_recursive(), and plenty others, need to
first initialize the sehandle with SelabelInitialize().

I wish there were a better solution, but early init doesn't actually
want this handle initialized, so that is a valid use case.  Ueventd
needs to initialize this before fork()'ing, so lazy initialization is
not universally acceptable either.  Likely we won't have other
children that fork() then exec() init again, so this should be okay.

Bug: 62875318

Test: init unit tests
Test: sailfish creates directories with correct SELabel after wipe
Change-Id: I6de937604a060e18945427418f15b90e0b9d5c37