sys.process.Process isAlive for 2.12 to match java8 lang.Process

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Very low hanging easy request to add support to poll a sys.process.Process for "alive-ness" to match the ability of java.lang.Process isAlive method.
It can be very very useful to have a non-blocking way to at least periodically poll that a process is done.
We'd like to make use of sys.process.Process more reactive for use with akka actors without the blocking of the waitFor which defeats reactive principles.
Since java8 is required for scala 2.12, then it should be quite easy to add an equivalent to java8 Process isAlive method into the sys.process.Process.

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Imported From: https://issues.scala-lang.org/browse/SI-9454?orig=1
Reporter: craig bordelon (bord)
Affected Versions: 2.12.0-M2

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@SethTisue said:
I believe a pull request with this change (targeting 2.12) would be welcome.

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@lrytz said:
scala/scala#4875