vbox: save the guest FPU state before 'longjmp()'

'longjmp()' restores the (partial) FPU state saved by 'setjmp()', so it's
necessary to save the guest FPU state before calling 'longjmp()'.

Fixes #1282
vbox: always transfer FPU ownership back to the VM

When the 'Mtd::FPU' flag is set during the registration of a
virtualization event handler, it must also be set whenever the event
handler returns.

Fixes #1283
vbox: set the 'CPUM_CHANGED_GLOBAL_TLB_FLUSH' flag

Fixes #1281
dde_kit: panic does not return

The attributes enables to use panic as final statement in branches of
functions declared to return.
vfs: use 256-byte buffer in log file system

128 bytes is not much for logging-output line length esp. when also
counting the color sequences of PDBG() and friends.
libc: do not zero errno on success in vfs plugin

The manpage to errno tells the following story:

The <errno.h> header file defines the integer variable errno, which is
set by system calls and some library functions in the event of an error
to indicate what went wrong. Its value is significant only when the
return value of the call indicated an error (i.e., -1 from most system
calls; -1 or NULL from most library functions); a function that
succeeds is allowed to change errno.

Valid error numbers are all nonzero; errno is never set to zero by any
system call or library function.
base: document specifics of Slab::alloc()

Also removed unimplemented overload of alloc() with no parameters.
dde_linux: make drivers self containing

Ported drivers list and extract all needed source files. This decouples
ports according to contrib sources and also enables us to revert lxip to
Linux version 3.9, while staying with 3.14 for usb.

Fixes #1285
fixup "dde_linux: make drivers self containing"

missing patch file
libports/sdl_image: enable XPM, add include/SDL

Some SDL applications expect the SDL_image headers in include/SDL to be
reachable without the SDL/ prefix. This patch adds the corresponding
search path. Furthermore it enables support for XPM images.
rpi: add buffered mode to framebuffer driver
libports/sdl: fix keycode for return key
fixup: "base: Genode's dynamic linker"

Re-order initializer list for dependencies that are by multiple ELFs. Make sure
to be these in front of the initializer list.