Rename x*() wrappers to mvwm_*()

Due to GNU Readline (yay, thanks GNU!  Grr) exporting xrealloc, and friends,
MvwmConsole ends up going mental with the wrong xrealloc() version.  Since
the nice folks at GNU Readline don't consider this a bug (huh?), rename
these wrappers to mvwm_* instead.