qspawn: remove some anonymous subs for psgi_qx

qspawn: remove some anonymous subs for psgi_qx

By passing a user-supplied arg to $qx_cb, we can eliminate the
callers' need to capture on-stack variables with a closure.
This saves several kilobytes of memory allocation at the expense
of some extra hash table lookups in user-supplied callbacks.  It
also reduces the risk of memory leaks by eliminating a common
source of circular references.