apache2: wait for server to start/stop/restart

Change start, stop, and restart functions in apache2 init script to return only
after completion (i.e. the server has started/stopped, not just received a kill
signal). Starting and stopping the server in quick sucession results in an error
because the server will attempt to stop before it has had time to start and vice
versa.

Signed-off-by: Adam Chappell <adam.chappell@ni.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>