Linux has had IP_MTU since 2005, don't force it.

ok beck@ miod@ tedu@ deraadt@
normalize set/getsockopt usage.

Remove the remaining random casts on optval. Fixups for this can be handled by
the portability layer all in once place.

Remove remaining fake socklen_t unions, though beck@ points out that this also
removes support for socklen_t changing its length at runtime. RIP.

ok tedu@ beck@ miod@ deraadt@
memset like a normal human.

ok beck@ tedu@ miod@
remove superflous gettimeofday wrapper.

ok beck@ tedu@ miod@ guenther@ doug@ deraadt@