init: use do_exec() for init_user0

Similar to what installkey used to do, init_user0 forks and
synchronously waits for vdc to return.  This is dangerous to do in
init however as init also processes properties from a single thread.

I'm not aware of any specific issues that this is currently causing,
but it's a good preventative measure to match what installkey does and
use do_exec().

Test: Boot bullhead, see that init_user0 still happens
Change-Id: I853c61594fe3d97e91bbb2319ebddf2bbe80d457