The only debugger command which uses this value currently is gdb. And
even there, it doesn't look mandatory. The alternative call using "-p"
option does not require the program name. The manual doesn't say if
calling with the program name has any advantage (but I don't see why it
would, the PID is enough to find a process). Just in case, I leave the
prog_name parameter (because it's easier to make a parameter useless
than changing a libgimp* API) but simply allows setting it to NULL.