Remove "Max Threads per Split Point" UCI option

Experimental patch to verify if drop of nps
in endgames at very long TC is due to this.

Suggested by Ronald de Man.

bench: 7451319
Split also if no slaves are found

Because we test for available slaves before
entering split(), we almost always allocate a
slave, only in the rare case of a race (less
then 2% of cases) this is not true, but to
special case this occurrence is not worth
the added complexity.

bench: 7451319
Fix a crash under MSVC

Using memset on a std::vector is undefined behavior,
so manually init all the data memebers of LimitsType.

Bug intorduced in 41641e3b1eea0038ab6984

No functional change.
Increase max threads to 128

Thanks to std::bitset we can easily increase
the limit of active threads above 64.

Thanks to Lucas Braesch for pointing at the
correct solution of using std::bitset.

No functional change.
Fix a typo

Spotted by Isaac Haïk Dunn.

No functional change.
Fix a subtle bug in UCI options printing

We want all the UCI options are printed in the order in which are
assigned, so we use an index that, depending on Options.size(),
increases after each option is added to the map. The problem is
that, for instance, in the first assignment:

o["Write Debug Log"] = Option(false, on_logger);

Options.size() can value 0 or 1 according if the l-value (that
increments the size) has been evaluated after or before the
r-value (that uses the size value).

The culprit is that assignment operator in C++ is not a
sequence point:

http://en.wikipedia.org/wiki/Sequence_point

(Note: to be nitpick here we actually use std::map::operator=()
 that being a function can evaluate its arguments in any order)

So there is no guarantee on what term is evaluated first and
behavior is undefined by standard in this case. The net result
is that in case r-value is evaluated after l-value the last
idx is not size() - 1, but size() and in the printing loop
we miss the last option!

Bug was there since ages but only recently has been exposed by
the removal of UCI_Analyze option so that the last one becomes
UCI_Chess960 and when it is missing engine cannot play anymore
Chess960.

The fix is trivial (although a bit hacky): just increase the
last loop index.

Reported by Eric Mullins that found it on an ARM and MIPS
platforms with gcc 4.7

No functional change.