ng_ksocket: fix accept(2)
- Provide listen upcall and set it on NGM_KSOCKET_LISTEN
- Mask EWOULDBLOCK on NGM_KSOCKET_ACCEPT
Reviewed by: afedorov
Differential Revision: https://reviews.freebsd.org/D42637
PR: 272319
PR: 275106
Fixes: 779f106aa169256b7010a1d8f963ff656b881e92