nova: show error message for un-handled exception

Fixes #938
Call exit handlers before notfying the parent

Fixes #941.
libports: add SDL_net

Fixes #936.
pthreads: return error numbers

pthread functions are supposed to return error values instead of setting
'errno' and returning -1.

Fixes #856.
libports: use SDL's SDL_cdrom dummy implementation

Fixes #934.
libports: use Timer_session as SDL's timer backend

A timer session is now used instead of a jiffy counter. This way, libSDL
can use a time source that is not bound to the granularity our libc's
nanosleep implementation. Currently, the granularity of nanosleep is in
the order of 10 milliseconds, which is far to coarse for the use of
SDL-using applications such as DosBox.

Fixes #934.
nova: fix size of virtual address space for 64bit

Fixes #933