The swab() function copies n bytes from the array pointed to by
from to the array pointed to by to, exchanging adjacent even and
odd bytes. This function is used to exchange data between machines that have
different low/high byte ordering.
This function does nothing when n is negative. When n is positive
and odd, it handles n-1 bytes as above, and does something unspecified
with the last byte. (In other words, n should be even.)