Per POSIX, O_EXCL is meaningful only together with O_CREATE. MemMapFs
OpenFile returned ErrFileExists whenever O_EXCL was set on an existing
file, so reopening a file with O_RDWR|O_EXCL failed even though no
creation was requested. Guard the existence check with O_CREATE so that
O_EXCL without O_CREATE opens the existing file, matching os.OpenFile.
Fixes#415
