linux(4): Implement CLONE_CLEAR_SIGHAND option bit.

CLONE_CLEAR_SIGHAND is designed to reset all signal handlers of the child
not set to SIG_IGN to SIG_DFL.

Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D31481
MFC after: 2 weeks

(cherry picked from commit c5fc9fe7f36a742e567173b2e3d65252140013d1)