halcmd: add '-i' flag to waitusr

if a component already exited, dont fail the waitusr and hence the
halcmd exit value.

$ halcmd waitusr foo
<commandline>:0: component 'foo' not found
$ echo $?
1

$ halcmd waitusr -i foo
$ echo $?
0
tests/edge: done fail the test if halsampler already did her job

it is ok if halsampler exited early. It is not ok to shutdown the script
if halsampler was not done at this point.

another stab at nailing down runtests failures involving halsampler