This conceptually reverts commit 1fdc161d
and reintroduces an option for srw_lock to wrap a native implementation.

The srw_lock and srw_lock_low differ from ssux_lock and ssux_lock_low
in that Slim SUX locks support three modes (Shared, Update, eXclusive)
while Slim RW locks support only two (Read, Write).

On Microsoft Windows, the srw_lock will be implemented by SRWLOCK.
On Linux and OpenBSD, it will be implemented by rw_lock and the
futex system call, just like earlier.
On other systems or if SRW_LOCK_DUMMY is defined on anything else
than Microsoft Windows, rw_lock_t will be used.

ssux_lock_low::read_lock(), ssux_lock_low::update_lock(): Correct
the SRW_LOCK_DUMMY implementation to prevent hangs. The intention of
commit 1fdc161d seems to have been
do ... while loops, but the 'do' keyword was missing. This total
breakage was missed in commit 260161fc
which did reduce the probability of the hangs.

ssux_lock_low::u_unlock(): In the SRW_LOCK_DUMMY implementation
(based on a mutex and two condition variables), always invoke
writer_wake() in order to ensure that a waiting update_lock()
will be woken up.

ssux_lock_low::writer_wait(), ssux_lock_low::readers_wait():
In the SRW_LOCK_DUMMY implementation, keep waiting for the signal
until the lock word has changed. The "while" had been changed to "if"
in order to avoid hangs.