kbdmux(4): Make callout handler mpsafe.

Both callout and taskqueue now have drain() routines not requiring
external locking. It allows to remove TASK flag and manual drain,
so the only thing remaining for lock to protect inside the callout
handler is ks_inq_length zero comparison, that can be lockless.

MFC after: 2 weeks

(cherry picked from commit e5018628e76a27e0f61ca03e2aa2247b3c62a158)