signal/signalfd: Add support for SIGSYS

I don't know why signalfd has never grown support for SIGSYS but grow it now.

This corrects an oversight and removes a need for a default in the
switch statement.  Allowing gcc to warn when future members are added
to the enum siginfo_layout, and signalfd does not handle them.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>