Adjust prototypes of xmalloc(), xrealloc() and xstrdup()

Adjust prototypes of xmalloc(), xrealloc() and xstrdup()

SylixOS defines its own versions of these functions in its standard
library, which collide with ours. However, their prototypes make
more sense, because they follow the prototypes of the non-x versions
in the C standard, so there is no harm in following them.