* net.cc (cygwin_getsockopt): Allow NULL optval.
* path.cc (mount_info::read_cygdrive_info_from_registry): Don't write cygdrive
to registry if it doesn't exist.
* path.cc (mount_info::conv_to_win32_path): Avoid returning error if cygdrive
== '/'.
(mount_info::cygdrive_win32_path): Return 0 if invalid cygdrive path.