Round out SYSCTL macros to the full set of fixed-width types

Add S8, S16, S32, and U32 types; add SYSCTL*() macros for them, as well
as for the existing 64-bit types. (While SYSCTL*QUAD and UQUAD macros
already exist, they do not take the same sort of 'val' parameter that
the other macros do.)

Clean up the documented "types" in the sysctl.9 document. (These are
macros and thus not real types, but the manual page documents intent.)

The sysctl_add_oid(9) arg2 has been bumped from intptr_t to intmax_t to
accommodate 64-bit types on 32-bit pointer architectures.

This is just the kernel support piece; the userspace sysctl(1) support
will follow in a later patch.

Submitted by: Ravi Pokala <rpokala@panasas.com>
Reviewed by: cem
Relnotes: no
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D4091