SI-4602 Make fsc absolutize source file names

The fsc server was using a path supplied by the client to turn things
like class path values into absolute paths. But it wasn't absolutizing
the source file names supplied to the compiler which lead to SI-4602.
This commit adds that absolutizing bit and cleans the logic up a bit
so that the settings object isn't told a path that it already knows.

A test is included that simulates changing directory by forcing two
different -current-dir settings on two different compile sessions on
the same server process.
SI-4602 Disable unreliable test of fsc path absolutization

The included test for fsc path absolutization almost certainly has
the same reliability problem as a similar test that was disabled in
https://github.com/scala/scala/pull/1985 . Disabling the test until
I can figure out a reliable way to test fsc in an our continuous
integration environment.
Merge pull request #1973 from JamesIry/SI-4602_2.10.x

SI-4602 Make fsc absolutize source file names