unRegisterWithParent called log.Panic when a file's parent directory was
no longer registered (for example when the parent was removed first),
crashing the whole program from an ordinary Remove call. Return a normal
error instead so Remove, RemoveAll and Rename can surface it to the
caller. The now-unused log import is dropped.
Fixes#388
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
dualfroz
upstream