Commit b1b5de4 switched from enabling select physics flags to setting
those flags and clearing all others. Unfortunately, flag PF_USES_THRUST
was omitted from the enabled list, so it was disabled on reset. It is
required to let player ships move.
Reported-by: Sottises <https://github.com/dxx-rebirth/dxx-rebirth/issues/347>
Fixes: b1b5de4297 ("Additional safeguard for bug #306")