xext: Fix shmint.h to not use headers outside of sdk_HEADERS

xext: Fix shmint.h to not use headers outside of sdk_HEADERS

shmint.h is part of sdk_HEADERS, and so can't use anything not
included in sdk_HEADERS.

busfault.h includes dix-config.h which is not. Leave the use of
struct busfault in shmint.h and move the include of busfault.h to
shm.c.

protocol-versions.h is not part of sdk_HEADERS, so instead of using
that, just use XTRANS_SEND_FDS to choose whether to expose the fd
passing requests directly.

Reported-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Keith Packard <keithp@keithp.com>
Tested-by: Knut Petersen <Knut_Petersen@t-online.de>
Reviewed-by: Julien Cristau <jcristau@debian.org>
v2: also avoid using protocol-versions.h