Multiple-instance support.

Multiple-instance support.

Write PID file to FG_HOME, use this to detect multiple launches.
When this situation is detected, set a marker property and place various
objects into read-only mode, such as the NavCache and TerraSync.

PID file is created using open+unlink semantics on POSIX, and
DELETE_ON_CLOSE on Windows, so it will be removed when fgfs exits,
even if killed or crashes.