The two POSIX style crash handlers (OSX and X11) now remove their signal
handlers when they are destroyed.
Additonally if they are called while no OS singleton is set, they will
simply abort(). This should not happen now that they remove themselves,
but if a future change seperates OS object and crash handler lifetimes,
this may be easier to report/debug than hanging on SIGSEGV.
(cherry picked from commit 653b832422)
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
