Fix an overflow in getsockopt(). optval isn't big enough to hold
sbintime_t.
Re-introduce r255030 behaviour capping socket timeouts to INT_32
if they're too large.

CR:	https://phabric.freebsd.org/D433
Reported by:	demon
Reviewed by:	bde [1], jhb [2]
MFC after:	2 weeks