Added DRAW command dsetsignal, allowing to reset OSD signal handler with either armed...

Added DRAW command dsetsignal, allowing to reset OSD signal handler with either armed or disabled FPE handler.
If called without arguments, it sets FPE handler only if environment variable OSD_FPE is defined (with value different from 0).
On start, DRAW calls dsetsignal to set FPE signal if CSF_FPE is defined.
Test bugs fclasses bug6143 uses dsetsignal to set FPE handler unconditionally before test execution.