Remove the re-entry check from do_call_back(), IPX (Reinhard) have
problems with it.
do_call_back() must be reenterable, but I am worrying about the
case where it is re-entered by the VESA switch, after which
dosemu will be frozen while callback is processing. Probably the
VESA driver have to wait if any callback is active?


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@791 7758b857-130e-0410-ab9c-d8735a528d18