Replace <sys/param.h> with <limits.h>

Replace <sys/param.h> with <limits.h>

This patch is from Theo.  I helped verify that removing <sys/param.h>
doesn't change anything.

This produces the same binaries before and after with clang 3.5 and gcc
4.9.4 on amd64.  There is a slight difference in the way it is generated by
our old gcc 4.2.1 despite the fact that the preprocessed input is almost
identical.

ok deraadt@