* path.cc (mount_info::read_cygdrive_info_from_registry): Use the user prefix

* path.cc (mount_info::read_cygdrive_info_from_registry): Use the user prefix
if it exists.
* sync.h (sync::init_lock): Declare new static member.
(sync::init()): Declare new static function.
* sync.cc (sync::init): Define.
(sync::init): Lock attempt to initialize a muto to stop multiple threads from
colliding.
* dcrt0.cc (dll_crt0_0): Initialize muto environment.