Don't use SA_NODEFER because it doesn't work as documented, at least, for
the time being. Explicitly unblock the faulting signal instead.
Unblock async signals in instremu.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@1275 7758b857-130e-0410-ab9c-d8735a528d18