RFR: 8042088: Sjavac spawns external processes in a unnecessarily complex and platform dependent way

[Andreas Lundblad]

Hi compiler-dev,

Please review this patch which addresses JDK-8042088.

- Description:
This patch is a refactoring and cleanup of the code that spawns the external process for running the sjavac server.

Since the fix of JDK-8048457 a server is never spawned as a separate thread. (background=false -> Direct use of SjavacImpl, background=true -> Server in separate JVM). This patch removes the (now dead) code that spawns the server in a thread.

The patch also fixes a small race condition in PortFile.

Tested:
- All langtool tests pass
- make images runs without errors
- Giving a bad launch command (sjavac=...) results in correct error message
- Bad server executable (that never writes values to the port file) results in correct error message

- Link to webrev:
http://cr.openjdk.java.net/~alundblad/8042088/webrev.02/

- Links to bug report:
https://bugs.openjdk.java.net/browse/JDK-8042088

-- Andreas