gcore: Forward pending signals when detaching from the target.

Otherwise gcore's ptrace attach operation can race with delivery of a
signal and cause it to be lost.

In collaboration with: Suraj Raju <sraju@isilon.com>
Reviewed by: bdrewery
Approved by: re (gjb, kib)
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division