Fix entry of deferrals

XBoard suffixes deferred promotions with an = sign, but GNU Shogi's
internal move generator to which the input move is compared doesn't.
Now we strip off the promochar if it is an =.
Statify genmove.c::DropPossible and ::LinkMove.
Rename (B)algbr parameter "flag" to "flags".
Do not declare "ahead" 3 times.
Stop using "false" for bitfields.
Statify more.