Allow booting with $_xms=(off).
dosemu always maps HMA, regardless of the $_xms setting. This check does
nothing but prevents dosemu from booting with $_xms=(off).
Unfortunaltely Win98/DOS hates our XMS driver, so possibility to disable
it, must work.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@510 7758b857-130e-0410-ab9c-d8735a528d18