init: add exec_start command

Exec services may also want to set other service flags such as
priority.  Instead of expanding the exec syntax to handle this, create
a new command, exec_start, that will treat an existing service
definition as an exec service.  The new exec_start command will start
the service then halt init from executing further commands until the
service has exited.

This change additionally encapsulates the waiting_for_exec logic into
ServiceManager and removes the ambiguous 'bool' return value from
Reap() which previously indicated if a Reaped service was an exec
service or not.

Bug: 36511808
Bug: 36102163
Test: Bullhead boots, services run with exec_start as they do exec.

Change-Id: I44f775cf1c1dd81d5c715f44fdc150c651a2c80a