Merge common rmdir, mkdir, and rename MFS code between SFN and LFN.
Fix problem with opening files in the root directory.
Honour read only drives in the LFN code.
Fixes #1426126 (LFN problem with current  DosEmu)


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@1530 7758b857-130e-0410-ab9c-d8735a528d18