recvmmsg's context comprises data needed by the system call. Setup can be
expensive, particularly for many buffers.
NOTE: Unlike most system calls involving Iobufs, the lo offset is not respected.
Instead, the Iobuf is implicity reset (i.e. lo <- lo_min and hi <- hi_max) prior
to reading and a flip_lo applied afterward. This is to prevent the memory-unsafe
case where an iobuf's lo pointer is advanced and recvmmsg attempts to copy into memory
exceeding the underlying bigstring's capacity. If any of the returned Iobufs have
had their underlying bigstring or limits changed (e.g. through a call to
set_bounds_and_buffer or narrow_lo), the call will fail with EINVAL.