sort: tune struct_merge_node slightly

sort: tune struct_merge_node slightly

* src/sort.c (struct merge_node): 'lock' is now the actual lock,
not a pointer to the lock; there's no need for indirection here.
Make 'level' unsigned int instead of size_t, since it is a
bit-shift count; also, move it next to a bool so that it's more
likely to take less space.  All uses changed.
(sortlines, sort): Spell out initialization instead of using an
initializer.  This makes the initializer a bit easier to understand,
and avoids unnecessary stores into the spin lock.