Add a sysctl, regression.sonewconn_earlytest, which when options
REGRESSION is enabled, allows user space to dictate that sonewconn()
should skip it's "skip the hard work" check to see if the listen
queue is full, and instead proceed with allocation of a socket and
trimming of the overflowed queue. This makes it easier to test the
queue overflow logic.

MFC after: 1 month