Rearrange the implementation of bounded sysctl. The primitive

Rearrange the implementation of bounded sysctl.  The primitive
functions are sysctl_int() and sysctl_rdint().  This brings us back
the 4.4BSD implementation.  Then sysctl_int_bounded() builds the
magic for range checks on top.  sysctl_bounded_arr() is a wrapper
around it to support multiple variables.
Introduce macros that describe the meaning of the magic boundary
values.  Use these macros in obvious places.
input and OK gnezdo@ mvs@