shared: Make xalloc.h stand alone

Make fail_on_null static inline and put it in xalloc.h so we can use the
header exclusively instead of having to link with the library for it.

This is so we can use xalloc in places (like the RDP backend) without
having to bring in libshared.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>