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.
Add a puaListBox which can show the contents of a log-buffer. Currently only two buffer sources are supported more to follow (and the dialog XML updates).
puObject only provides float, not double, which causes precision/rounding
issues with some numerical values (try "114.2").
Work around: obtain string value, and manually convert with proper double
precision.
