Unite sp_search() and sp_search_pv()

Unite sp_search() and sp_search_pv()

Also introduce a new rule:
In sp_search() always must hold: sp->alpha < sp->beta
Should fix some rear but very nasty races

To keep everything in sync, search() is also modified to
obey this rule. Because this affects only PV-nodes, should
have zero meaning to speed.

No functional change in fake mode

Regression test after 854 games
Mod vs Orig 433 - 421, no crashes.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>