Switch NetBSD from paccept(2) to accept4(2)

Summary:
NetBSD 8.0 will ship with accept4(2) in libc wrapping paccept(2).

This change reduces needless difference with other platforms.

Older versions of NetBSD will not be supported.

No functional change.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, emaste, labath, clayborg

Reviewed By: emaste, labath, clayborg

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D30288

llvm-svn: 296070