Fixes to unix builtin:
- Dont strip the trailing slashes for the DOS directory path, as this
path may consist of only that slash (second part of #1152829)
- Dont terminate if the linux path we wanted to execute, doesn't exist.
For the DOS path, figuring out the existance is more difficult, so left
as is for now.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@998 7758b857-130e-0410-ab9c-d8735a528d18