* dir.cc (mkdir): Check path for writability.
(rmdir): Ditto. Remove check for special directories.
* path.cc (path_conv::check): Set PATH_RO for virtual devices. Set error if
read-only and asked for writability.
* path.h (pathconv_arg): Add PC_WRITABLE.
(path_types): Add PATH_RO.
(path_conv::isro): Add (currently unused) check for read-only filesystem.
Return "ENOSHARE" when we know a share doesn't exist.
* include/sys/mount.h: Add MOUNT_RO flag.
* autoload.cc (WNetGetResourceInformationA): Import.
* fhandler_netdrive.cc (fhandler_netdrive::exists): Detect actual existence of
remote system rather than always assuming that it exists.
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